Can all the colours in the ad that are RGB be expressed in CYMK?
No.
And furthermore, it’s more complex than you realize. There is more than one way to represent a particular RGB color in CMYK. In order to convert an RGB color to CMYK, you must know such factors as what kind of printing press will be used, how much ink can be applied to the paper before it will smear, and so on.
This is very complex stuff; creating a good color separation that works well on the kind of paper and press that will be used requires a good understanding of color theory, color separation, and prepress.
You are attempting to do something that is extremely complex and requires specialized knowledge. You are in a position where you do not even understand how complex the task is.
I suggest two things:
1. Get a prepress professional to prepare the ad for press. This is what they get paid for. It’s expensive, but it’s a standard part of professional ad creation. You do not have the skills necessary to do what you need to do. You are a designer, not a prepress technician; do not attempt to do prepress.
2. learn a little bit about prepress. It is not necessary for you to be able to do it yourself; in the world of professional advertising, almost no designers actually do the prepress. You are paid for your design skills, not your prepress skills. But a little bit of basic knowledge about prepress and printing will save you from making mistakes like the one you have made, and will make your jobs go much easier (and less expensively!) in the future.
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