Red, Green, and Blue are the Primary Colors. They are the “irredicible components” of white light. All other colors are simply combinations of Red, Green and Blue.
Cyan, Magenta, and Yellow are “Complements” to Red, Green, and Blue. You’ll note that they lie opposite the Primary Colors on our Color Wheel:
The reasons for this are remarkably simple and easy to comprehend!
Red, Green, and Blue, being parts of White Light, are “additive.” This means that as you add brightness to them, they approach White.
In the above graphic, Red, Green, and Blue are added to each other (using the Linear Dodge Blending Mode, just in case you’re in to that sort of thing). You can see that where they all intersect our result is White. Where only two of the Primary Colors intersect is their Complement: Red and Blue produce Magenta, Red and Green produce Yellow, and Green and Blue produce Cyan.
The Complementary Colors, Cyan, Magenta, and Yellow, are “subtractive.” As you add them to each other, you approach Black (the absence of Light, therefore, the absence of Color).
This is most useful in the printing disciplines. We use inks to absorb light (subtracting it), and the residual reflected light is what you see as the print! The science beneath the matter far exceeds an introductory tutorial, but it is important to understand that C, M and Y are subtractive and are not Primary Colors.
Note: In this case, I used the Linear Burn Blending Mode to demonstrate the subtractive effect.
You may be wondering why the “K” (for Black) in CMY if they add up to Black in the first place? The reason is that inks-both the dyes or pigments used for color and the solutions used to suspend the dyes or inks (the “vehicles”) are imperfect. If you just add amounts of C, M and Y ink together in the real world you won’t end up with black, you’ll end up with a very runny dark brown mess. It’s also very inefficient to use large amounts of ink on print media.
Black ink solves a number of problems associated with producing dark tones with subtractive color.
On your own study:
Black Point Compensation