Sunlight looks white — but it isn't. It's every colour in the rainbow travelling together at once. Mix all those colours of light together and you get white. Split them apart and you get a rainbow.

Each transparent coloured paddle works as a filter. It doesn't add colour — it removes it. The yellow paddle absorbs blue light and lets red and green through. Red and green light together look yellow. That's why the paddle appears yellow: everything else has been taken away.

Hold up a red paddle: it absorbs both blue and green, leaving only red. Now overlap the yellow and red paddles. The yellow has already removed blue. The red then removes green from what's left. Very little light survives both filters — and what does come through is darker and a different colour. 

Add more paddles and each removes more of the remaining light. Stack enough and you get black: all the light gone, nothing left to reflect.

Scientists call this subtractive colour mixing — adding more material makes it darker, not brighter, because each filter subtracts colour from white light. This is exactly how a printer works: three inks (cyan, magenta, yellow) each absorb different colours from white paper to build up any colour you can imagine.

Your screen works the opposite way — it adds red, green, and blue light together from darkness. 

You can see that additive system in action in [What Colour is Your Shadow], where mixing coloured lights produces unexpected and beautiful results. Paddles and printers subtract from white. Screens add to black. Two completely different systems — both producing every colour you can imagine.

But here's the twist — your phone screen mixes colour in the completely opposite way to these paddles. And mixing all the screen colours together makes white, not black. How? 

Find out in The Crazy Scientist Lab