There are two completely different ways to make colour in lighting, and confusing them is one of the most common beginner mistakes. Additive mixing combines coloured light; subtractive mixing removes wavelengths from white light with filters.
Additive — the LED way
Start with darkness and add coloured light. Red + green + blue at full climb toward white. This is how every RGB / RGBW LED fixture works, and how your phone screen works. More emitters means brighter and closer to white.
- Red + Green = Yellow
- Green + Blue = Cyan
- Red + Blue = Magenta
- All three at full = White
Subtractive — the gel & CMY way
Start with white light and take wavelengths away. A red gel absorbs everything except red. Stack a second gel and you remove even more — stack enough and you approach black (and a very hot gel). The CMY colour-mixing flags in moving heads work this way: cyan, magenta and yellow filters slide into a white beam.