As opposed to the mixing of light (“additive color mixing”), subtractive color mixing occurs only when combining dyes, pigments, or other objects that absorb and reflect light. For example, you could shine white light through two colored filters or gels. Or, you can reflect light from colored surfaces. If you shine white light on a blue wall, the wall absorbs red and green but not blue. So if we know what colors are reflected we know the color of the object.

The primary pigments or dyes are magenta, which reflects blue and red light but absorbs green; cyan, which reflects blue and green but absorbs red; and yellow, which reflects red and green but absorbs blue. These are the same colors as the secondary colors obtained when mixing light. Note that if we combine magenta, cyan, and yellow, the red, green, and blue are all removed, leaving nothing; that is, black.

Blue is light in the range of 400 to 500 nanometers. Green is roughly 500 to 560 nanometers. Yellow is 560 to 590 nanometers, orange 590 to 620 nanometers, and red beyond 620 nanometers. It may seem surprising that yellow filter transmits green, yellow, and red. But the eye is not very sensitive to red in comparison to green and yellow. That’s the same reason that the transmission of the blue and indigo filters beyond 660 nanometers does not impact what the eye sees. Finally, note that purple is a mixture of blue and red.