RGB Colours
Red, Green, and Blue (RGB) are the three main primary colours of a computer monitor.
There are two main colour profiles: additive and subtractive.
RGB is an additive profile because you create new colours by adding the (red, green, blue) colours together.
Green and Blue together = Cyan
Blue and Red together = Magenta
Red and Green together = Yellow
Red, Green and Blue together = White
The range of colours RGB gamut can produce is astronomical, because of how it mimics our eyes ability to see colour.
In our eyes, we have essentially three cone cells that are able to see the primary colours red, green and blue. By mixing these colours together at different intensities, our brain is able translate this into a multitude of different colours.
The main factor here is light. When you are printing, light isn't generated, it's reflected off of the substrate.
Therefore we don't add, we subtract when printing in CMYK; as colours need to be adjusted to reflect what a light source produces.
CMYK Colours
CMYK : Cyan, Magenta, Yellow and Black. These four colours of toner or ink are blended to create colours on items that reflect light on the substrate.
Unlike RGB where colours are created by adding two or more together; CMYK is a subtractive colour profile, so you remove a primary colour of light from any white light that hits it.
Cyan ink reflects green and blue light, but not red
Magenta ink reflects blue and red light, but not green
Yellow ink reflects red and green light, but not blue
To produce the three primary colours, you have to think in reverse.
By combining cyan and magenta, you only reflect blue light
By combining magenta and yellow, you only reflect red light
By combining yellow and cyan, you only reflect green light
Imagine you have a piece of white paper. It’s white because it reflects every wavelength of light. By using cyan, magenta and yellow in different proportions, you subtract specific colours of light to produce a wide gamut of colours.
Now, in theory, by combining cyan, magenta and yellow you should get black. But our eyes perceive colour the RGB way, meaning the CMY doesn’t produce a perfect result. The is where the black ink comes in; it will absorb all light and provide darker tones and rich black.
Range
Since the CMYK profile works on a subtraction theory, its colour spectrum is limited when compared to RGB. Vivid primary light colours (pure red, green and blue) will often appear less vibrant when converted to CMYK.
In the image, you can see the gamut of available colours with (most) RGB profiles, and those available with CMYK.
Keep your workspace in CMYK to ensure colours are kept within gamut.
Out of Gamut
If we take RGB red : R (255) G (0) B (0)
It's a bright vibrant red, but a warning symbol next to the colour shows it's outside CMYK gamut.
If we were to print this, the result would be muddy; as it's not a colour the printer can produce.
Stay in CMYK
Now create the same red but change to a CMYK reference : C (0) M (100) Y (100) K (0).
The warning symbol is gone and the printer would print the best red it possibly can.
Any colour will never reflect as on screen due to (99% of) screens working in RGB and also being a literal source of light behind/as each colour you see.
CMYK in Photoshop
Now we have established the importance of using a CMYK profile when printing, we need to know how to change to change over if required.
When creating a new document, make sure the Colour Mode is set to CMYK Colour.
Image > Mode > CMYK Color
CMYK in Illustrator
When creating a new document, make sure the Colour Mode is set to CMYK Colour.
File > Document Color Mode > CMYK Color
Illustrator CMYK palette
We also need to ensure the artwork colours are in CMYK.
Open the Swatch Window > Swatches Tab > Open Swatch Library > Default Swatches > Print.