How colour is made by light
Physics · Key Stage 2 · The School
White light is not one colour
Ordinary "white" sunlight is actually a MIXTURE of every colour of light, all travelling together. A prism can split that mixture apart. So can raindrops, making a rainbow. Both spread it out as red, orange, yellow, green, blue, indigo and violet. Nothing is being added by the prism. The colours were always there, simply overlapped.
Why things look coloured
An object looks a certain colour because it REFLECTS that colour of light back to your eye. It ABSORBS the rest. A red apple absorbs almost every colour except red. Red bounces off it, into your eye. A colour is not something an object makes. It is something the object does to the light already falling on it.
What coloured light changes
Shine only green light on a red apple, with no other light present. The apple now looks almost black. There is no red light present for it to reflect. It cannot reflect a colour that is not there to begin with. This single fact is what coloured stage lighting relies on. It changes how the very same costume appears from one scene to the next.
A red apple is tested under three different single-coloured lights, one at a time, in an otherwise dark room. Under red light, the apple reflects the red light back. It looks its normal bright red. Under green light, there is no red light for the apple to reflect. It looks almost black. Under blue light, the same thing happens again. It is almost black, for the same reason. The apple itself never changed at all through this whole test. Only what colour of light reached it changed. That decided everything about what colour it appeared to be.