The illumination of an RGB keyboard features about 16.7 million different shades of colors. The term RGB refers to the RGB color model: each color is defined by its red, green and blue components. Many computer systems work with a color depth of 8 bit which means that each of the three color components is assigned a number from 0 to 255. This results in 256 x 256 x 256 different possibilities, i.e., there are overall exactly 16,777,216 colors possible.
The options to configure the RGB illumination of a keyboard vary from brand to brand. Most manufacturer though provide their mechanical RGB keyboard with various illumination presets, while also making individual customization possible. This goes often as far as being able to change the color of each individual key.