You can do it either way.
If using a spot channel, you can control the amount of the spot ink by entering in a number for the solidity. If you want pure color, enter in 100. If you want color, but maintain all the tonal values, enter in 0. Don’t want either extreme? Play with the solidity settings until you get something you like. Then save as a Photoshop DCS 2.0 file (single file, color composite).
If you want to use a CMYK space, it’s pretty simple. Just empty out the Cyan and Yellow channels and tell the printer that Magenta prints as PMS Red (or whatever color you choose).
A couple caveats to this particular method is that if the rose uses any yellow or cyan colors, your magenta might not be as "rich" as you would like it (i.e., you’ll lose some image data when you delete your cyan and yellow channels).
A way to combat this is to take your entire image and convert it to grayscale. Select everything and then copy it to your clipboard. Convert to CMYK and delete all the values from the Cyan, Magenta and Yellow channels. Then paste your clipboard contents into the Black channel (you need to have a background layer/flattened image for this to work). This will give you a composite gray that will maintain all the tonal values and integrity of the entire image. Then select the parts of the rose you wish to have red, copy that to the clipboard and then paste it into the Magenta channel. You can save this file as a TIFF, EPS or PSD file (depending on what your printer/service bureau will accept).
Hope this helps.
BLUDVLZ, great thanks for the help. I have just a few questions to clarify. When you say "delete all the values from the Cyan, Magenta and Yellow channels" I’m not sure how, do I trash them?
When I make a selection of the rose form the black channel and paste it to the Magenta how can I paste it to the same location in the photo (it paste to the center). I bet there’s a trick to that.
Also, can I adjust the levels of the Mag and black channels separatly to adjust how much magenta appears in the rose?
Thanks for all the detailed help.
When you say "delete all the values from the Cyan, Magenta and Yellow channels" I’m not sure how, do I trash them?
Don’t trash the channels, do a Select All (with the channel selected) and hit delete.
When I make a selection of the rose form the black channel and paste it to the Magenta how can I paste it to the same location in the photo.
Keep your selection active when you paste into the desired channel.
