Light mixes the opposite way from paint. When colored lights shine on the same spot, their light adds up. This is additive mixing, and its primaries are red, green and blue light.
| Lights | Make |
|---|---|
| red + green | yellow |
| green + blue | cyan |
| red + blue | magenta |
| red + green + blue | white |
| no light | black |
Key idea
Lights add: each one brings more light, so mixtures get brighter. Red + green = yellow; green + blue = cyan; red + blue = magenta; all three = white.
Why it works
Our eyes have three kinds of color-sensing cells (cones), most sensitive to long, medium and short wavelengths of light. Red light mostly stirs the long ones and green light the medium ones. Together they stir both, the same way yellow light does, so we see yellow. Newton showed the other side of this: recombine all the colors from a prism and you get white light again.
Worked example: Red and green light
A red spotlight and a green spotlight overlap on a white wall. What color is the overlap?
- These are lights, so they add.
- Red + green light stirs the same cones as yellow light.
- The overlap looks yellow, and brighter than either light alone.
Answer: Yellow.
Common mistake
Expecting red and green light to make brown, as paint would. Paint subtracts; light adds. A quick check: is the color made by something that gives off light (a screen, a lamp)? Then think additive. Is it on a surface that reflects light (paint, ink)? Then think subtractive.