When you use CSS with your browser, the colours that you see are from the display emitting light. When the red, green, and blue primary colours combine to make a colour, they are additive: the more of each component there is, the lighter the colour. Which is why #FFF is white (or why white is #FFF, depending on your perspective).

Conversely, when you print, the colours that you see are from the light that is reflected from the printed surface. When the cyan, magenta, and yellow primaries combine to make a colour, they are subtractive: the more of each component there is, the darker the colour. cmyk(1, 1, 1, 0) should give black, but in practice, it’s closer to a muddy brown. That’s one reason why black is added as the fourth colour.^[9] Using black ink is also less expensive than using a triple quantity of coloured inks. Text is typically printed solely in black to avoid problems if the other three inks are not perfectly aligned. A ‘richer’ black, which might be used for example in a graphic, can be made by applying solid black over one or more other colours.

Figure 14. RGB and CMY

The relationship between RGB and CMY was first demonstrated by James Clerk Maxwell around 1860. Put simply, the printing primaries are the secondary colours of the transmitted light primaries, and vice-versa: cyan is blue plus green, or white minus red, and similarly for magenta and yellow. However, as the following figure shows, CMYK printing inks do not have the same gamut (i.e., colour range) as the sRGB colour space that is used for RGB colours on the web, and neither covers the full gamut of visible light.

Figure 15. sRGB and CMYK gamuts

These differences between sRGB and CMYK might affect your use of colour. Every device or process for printing that you encounter will have a way of representing RGB colours using CMYK, but that is usually by shifting colours so that out-of-gamut colours can be printed (and shifting nearly out-of-gamut colours so they do not look the same as the shifted colours). If your paged media will mostly be viewed on screen, possibly with some local printing by end users, then RGB colours may be best. However, if your paged media will be commercially printed, discuss with your printer whether to use RGB or CMYK for images, etc., and who will do the final conversion to CMYK. Preparing images for commercial printing is a complex subject that is beyond the scope of this tutorial.