Unlike screens that emit certain wavelengths of light, inks absorb certain wavelengths of light. Instead of primary colours or RGB, they use CMYK, which stands for cyan, magenta, yellow, and key (which is black). Cyan, magenta, and yellow are complementary colours to red, green, and blue, so it’s easy to combine CMY to create RGB.
Most printers use cyan, magenta, yellow, and black ink instead of the primary colours we learned in grade school – red, blue, and yellow. Why is that? Or, why not red, green, and blue (RGB), like most computer monitors do? The answer has to do with how light is absorbed/reflected by the ink on the paper compared to how light is emitted from a light or displayed on a screen.
What makes the colour different?
When light is emitted from a screen, it uses a system that’s called additive colour. This means that if you want the colour purple, you can emit red light and blue light and the light adds together. Or if you emit red, green, and blue, they all make up white light. The way black is created is by not emitting light.
With ink, it is almost the opposite. A coloured surface like ink absorbs one colour and reflects everything else. The reflected colours are what is visible. For example, cyan ink absorbs red light, which is why it’s visible as red’s complement (opposite) on the colour wheel. Magenta’s complementary colour is green and yellow’s is blue.
The problem with combining colours of ink is that if you add green and blue ink, it absorbs both of their complements (magenta and yellow), and not much light is reflected. This is known as subtractive colour, since adding more colours subtracts from the light that comes from the ink/paper. Combining red, green, and blue should make black.
So why do they use cyan, magenta, yellow, and black?
They are all relatively light colours, compared to the standard RGB format for screens. Since mixing colours on paper makes it darker by absorbing more light, it’s harder to create lighter colours out of darker ones. Mixing darker colours tends to look muddy.
Black is included because cyan, magenta, and yellow together don’t completely absorb all light. (They make more of a dark grey).
But most importantly, as complements to red, green, and blue, only two of cyan, magenta, or yellow are needed to make any RGB colour.
If you’re interested in the science behind it, the primary benefit of CMY is that each color overlaps two primary colors. Cyan overlaps blue and green, Yellow overlaps green and red, and Magenta overlaps red and blue. With these overlapping colors we can now begin subtracting lightwaves to create specific colors, for instance to create pure Red you would mix Yellow with Magenta. What you’re technically seeing when you’re painting is that the paint is now absorbing most of the Cyan spectrum and reflecting Magenta and Yellow back, which results in Red.
As a result, using CMYK colours lets your printer reach just as many colours as if it was using primary colours or RGB, while being able to reach both light and dark shades of everything more easily.