Your student is thinking of subtractive colors. RGB is additive colors, and these colors add together to white. This is mixing light (projected color).
Subtractive colors are the ones that use Yellow, although it is Cyan and Magenta that are the other components. This is reflected color.
Hope this is a good enough answer, although I hope Gernhot (sp?) comes in. He is the real color expert around here.
Don
Each RGB color must share different portions of the luminance (perceived darks and lights) spectrum. Since yellow’s luminance is too light then the red and blue channel’s would have to make up the difference. This would reduce the amount of colors you could attain in an RGB editing environment.
The RGB color space closely mimics the way the rods and cones in our eyes perceive color. Being that green is the lightest color of the three you’ll notice it has the greatest affect on contrast by just adjusting curves or the levels middle slider on each channel.
This is just my own observation but there are a number of color science sites that can explain it better.
IMO the student’s question is a good one – the answer
cannot be found by common sense.
This doesn’t mean that the previous answers are wrong,
but they might not be convincing for the student.
Indeed we could mix green by emitted yellow and blue light.
Now we can refer to the chromaticity diagram. One has
to believe its SYMBOLICAL meaning, but it’s not necessary to understand the background.
<
http://www.fho-emden.de/~hoffmann/gamuts08072002.pdf>
The horseshoe contour defines the human gamut. A triple
of light sources (primaries) delivers a triangle.
Its content shows the gamut of the device. This triangle should be as large as possible, but inside the horseshoe. Each primary should be efficient – high luminance for little energy.
This defines practically the CRT or TV primaries (sRGB, PAL).
What would happen if we replace green by yellow ?
Any mixture of yellow and blue is on a straight line between these (new) primaries (one has to believe this). Halfway is a very WEAK green – little saturation, near to gray.
Tim’s argument, essentially.
This says: it’s better to use red, blue and green instead of yellow. But then the mixed yellow is not the best.
It’s a compromise: gamut size, efficiency, technical constraints.
Best regards –Gernot Hoffmann
RGB and LAB colours are really models only pertaining to receptors in the human eye. In truth, neither with additive nor subtractive colour do mixes of colors create others. Think about colour expressed as wavelengths. Violet (purple) is at the end of visible light. It can’t be created by any mix of colours of longer wavelengths. The reason colour models work is that we don’t have receptors for all colours, so different colours look the same when they really aren’t. This was exploited in WWII by employing colour blind people to see Japanese snipers dressed in camoflage.
Think of colour models as a magic trick that works because we don’t perceive reality as it actually exists.
Dear Philo,
please don’t make the situation more complicated than it is already. No need to introduce Lab here.
Violet can be created easily as a metamer (mixture) of
red and blue. Here I would like to correct your opinion, if you don’t mind, because this was an extraordinary
discovery by Isaac Newton:
Violet is EITHER a spectral color OR a mixture of red
and blue. Newton invented the color wheel, and he intro- duced surprisingly an angle gap between violet and red
– about 100°- which is filled by metameric magenta (as
we would call this purple nowadays).
<
http://www.fho-emden.de/~hoffmann/prism16072005.pdf>
Best regards –Gernot Hoffmann
Gernot: you are probably right about not confusing things, but I think that the reason RYB seems more intuitive is that it is difficult for us to conceive of reddish green. The reason for this is that our preretinal nerves divide colours into Blue/Yellow and Red/Green pairs. (LAB model of colour) The very fact they do so is the reason our cortexes have a hard time conceiving of the mixes. Since our cones have roughly red/green/blue colour receptors, we use RGB as a model. So I think the reason we pick the particular models we do has to do with limitations of our sensory organs, rather than the reality of colour. If it weren’t for metamerism, we would have a hard time describing colours to others.
RYB was/is taught in grade schools as the primary colors.
Wouldn’t Blue light and Yellow light add up to white light?
This is indeed a very learned discussion and wonderful to see and participate in.
My naive reaction is that abstract spaces are wonderful things but have a major limitation: they are abstract.
When it comes to rendition and rendering an image there are major constraints. The most important of these is: what can the hardware do?
If the couating on a CRT is limited by the color values of the coating even the best abstract space will ultimately remain abstract.
This then become the wonderful engineering challenge of making it happen with a realistic, practical and pragmatic approach balancing abstract aims with material limitations in a productive environment.
It has to be do-able, real and economically viable.
In a word: great!
The primary colours of light and the primary colours of computer / TV monitors are entirely different. Why does a photocopier see red, but not light blue ? Hence the old "blue pencil" editing things ? Then look at your printer…. no green ink is there ? Very confusing issue… much research needed, and if Search Engines don’t work, your local Library or Evening / Extension Course will yield much more information than can be typed in one message here. P.S. Dont ask the Librarian where the "Self Help" Section is…. she or he will throw you a withering glance.
"…In truth, neither with additive nor subtractive colour do mixes of colors create others. Think about colour expressed as wavelengths. Violet (purple) is at the end of visible light. It can’t be created by any mix of colours of longer wavelengths…"
With all due respect, Philo, those statements are too blatantly false to go unrefuted. ALL colors result from the simultaneous stimulation of only three cone types—SHORT-,MEDIUM-, and LONG-wavelength types corresponding roughly to "blue", "green", and "red" sensations. All color sensations are the net response to mixes of these three sensors’ outputs. The sensation produced by a given cone type varies in intensity depending on wavelenghth but not in quality. For that cone type, a signal at peak spectral response is indistinguishable from one at twice its intensity at a wavelength where the sensitivity is half of peak sensitivity, etc. A wavelength has a unique color, but a color does not have a unique wavelength. Violet IS a mix of responses from red and blue sensors. That it is experienced at the short (blue) end of the spectrum is the result of a low-level resurgence in sensitivity of the "red" cones. Violet is, as Gernot has stated, nothing other than a tone of magenta (mixture of blue and red).
George
Wouldn’t Blue light and Yellow light add up to white light?
Since yellow light is a combination of red and green light, combining it with blue light would indeed produce white light.
Even though I’m quite comfortable with the concept of additive and subtractive primaries, when I see a museum or classroom demonstration of additives, with three over-lapping light sources, seeing red and green combine to make yellow still seems to defy common sense.
But my eyes tell me they ideed do.
It’s actually far more simple than what has been made out. In grade school they may SAY red, yellow and blue, but what those colors actually are are more magenta, yellow and cyan. Real "blue" is often called violet rather than cyan and the "red" can be a bit towards the yellow side of magenta.
A pure violet or "purple" light is a spectral line at a single wavelength in the violet range. As George said, our eyes have receptors that sense ranges, rather than specific wavelengths; they can’t tell that this light has a wavelength of X angstroms. The violet light would have a particular signature, being received at different levels in the three types of receptors, and our brains and/or optic nerves would use this signature to identify this as being the color that we associate with violet. Those receptors can be fooled, however, by being provided with a combination of different wavelengths that results in a similar signature and is therefore perceived as violet. The one way in which the pure violet light could potentially be distinguished from the false violet light would be if there is chromatic aberration in our perception of the light (i.e., our eyes [in combination with our eyeglasses, contact lenses, etc.] "fringe" colors, with reddish hues on one side and purplish hues on the other side, due to prism-like effects). In this case, the pure violet spectral line would experience no fringing and would be perceived as a single color, while the mixture of colors that appears to be a similar color would experience fringing due to refraction of the red and blue/violet components, so that receptors on one side would not "see" the same color signature as those on the other side.
demonstration of additives, with three over-lapping light sources, >seeing red and green combine to make yellow still seems to defy common >sense
Try doing it in Photoshop…. 🙂
Many Thanks to all that have contributed. I’ll pass on the information.
George: I’m not a fan of relativism. You define colour by human receptors. A meter measuring wavelengths of light would define it differently.
My point is that colour models used (in PS, gradeschool, etc.) relate only to human perception, not the reality of light transmission or absorption. By your definition, a person with red green colour blindness could accurately state that there is no such thing as red.
So I think colour models work for the same reason magic tricks work: they successfully fool people.
And if you eliminated the green channel and replaced it with yellow you’ve just thrown away your cyan color. Cyan is NOT in the Blue channel nor is it in the Red. The Blue channel (from a monitor’s stand point since that’s the only way we can visually edit electronic color) is made up of a specific pigment (wavelength) that can’t be physically mixed with other colors to get that hue.
I’m speaking from a paint mixing output perspective putting aside subtractive and additive for simplicity sake. That 255 Blue is like an intense version of a tube of Ultramarine or Royal blue you get at any hobby store. Watercolor and dyes being the most intense over acrylics and oils. Cerulean blue (an intense midtone cyan sky blue) is another pigment that can’t be mixed.
You have to buy that specific pigment. You can’t mix other blues to get that specific hue. However you can mix intense Cadmium or Lemon Yellow and intense Cyan to get monitor Green. If you eliminated green for yellow you’ve thrown out Cyan.
What I’ve often wondered is how much color can a monitor display compared to what has been theorized about all the colors the eye can see in the full spectrum wavelength of light.
I took a shot of the prism effect off a CD with my Fuji F10 digital camera. This shot <
http://www.pixentral.com/show.php?picture=1G6cqjRz9tJr4f64nC dEnjblqS5kr41>is VERY ACCURATE to what I saw in reality. It’s just a bit posterized in some areas of the color swath and not as intense, and the pastelish cyan was not as pronounced. The cyan actually resides in a very small thin section of the transition between cerulean blue and RGBgreen. It just took tilting the CD under the light to find it.
Try doing it in Photoshop…. 🙂
Simple to do
I know why I endure some of the sadder moments in the forum. This IMHO is verging on the (real or abstract) excellent!
Who’d have thought that such a simple question would require consideration of: – human perception
– materials sciences and limitations therein
– present working practices
– present theoretical models
– present practicable production
Brilliant!
Dear Gustavo,
in your last link (‘handprint’) I found this strange statement:
‘Theory of Color by Johann Wolfgang von Goethe – This is without a doubt one of the oddest "color theory" books available. The German poet and bureaucrat Johann Wolfgang von Goethe (1749-1832, pronounced "GURta") spent almost two decades of his maturity writing his collection of miscellaneous color notes a fat compendium of unfounded speculation, anecdotes from artist acquaintances, informal naturalist observations, and homebaked demonstrations with prisms, colored papers and staged lighting all published in 1810 as Farbenlehre ("Color Learning," translated into English in 1840).’
Never read such a nonsense. ‘Goethe’ isn’t pronounced ‘Gurta’ (because he wasn’t an Indian) and his book ‘Farbenlehre’ has to be interpreted deliberately, like here:
<
http://www.fho-emden.de/~hoffmann/prism16072005.pdf>
Goethe’s book is a profound compendium of the state of the art about color science at that time. But few people read it and few people read Newton’s ‘Opticks’.
Compared with Goethe and Newton I’m only a calibrationist. One has to respect great minds like Goethe and Newton.
Both are right, by their points of view, but Newton delivered the scientific basis for our actual color models.
Best regards –Gernot Hoffmann
I concur with Gernot, what utter nonsense. Goethe is a founding colour scientist, not an unfounded speculator. He wasnt right about everything, but he was a pioneer.
Mathias
Gernot
We may agree that, obviously the webmaster has not a very high opinion of Goethe (whose name I have no idea how to pronounce) as he calls him "poet and bureaucrat ".
Pity, because in general terms the site is rather good and the author has worked a lot in it.
Compared with Goethe and Newton I’m only a calibrationist
That leaves folks like me quite low in the echological pyramid of this realm. LOL!
Wormly (and lightheartedly) yours, as usual 🙂
I just wish someone would use this color science to come up with better monitor phosphor pigments than the standard P22.
But the EPA would probably put a halt to that.
Deebs: Gernot and George (I think) have written extensively on colour theory themselves, so I would probably weigh their comments more heavily than others. However the implicit issue is: does colour exist independently of human perception. If your definition depends on it being registered by a human, then they are right. The problem with not defining it by human perception, is that it becomes almost impossible to discuss, because most real colour is not single wavelengths (except if a few cases of excited ions) but a distribution of varying visible and invisible radiation wavelengths. Fortunately for Photoshop, we can largely disregard this and define most (but not all) colours in some colour space. The colours we can see, however, are a bit wider than most colour spaces, so for an artist, it is true that some colours need specific pigments to create them (rather than just mixing CMYK pigments). I think this was born out by some of the comments previously.
I would imagine that George and Gernot would disagree with most of what I have said as the term "colour" has already been defined by others and has been defined in terms of human vision. If you take that as your postulate, then they are right.
I don’t really see it as an either-or but consequential to context 🙂
Just for the record: some animals have only two colour receptor cell types and some have four (one more than we do). For these species, color wheels would be distinctly different. So back to the question asked: RGB but not RYB will describe colours that humans see.
Philo,
Then the eyes have it? 🙂
What an incredibly intricate mechanism, the eye!
George
