These remind me at school ( long ago now) we were shown a wheel of rainbow colours and when it was spun it looked a grayish whiteproving white light consisted of all the rainbow colours.

It's called Newton disc

http://en.wikipedia.org/wiki/Newton_disc

 

In CRT TV, LCD, LED, there are just red, green and blue diodes.

If we mixed fine sands of rainbow colours we would have a sort of whitish sand.

 

Actually, you would get black - it is the difference between additive (light) and subtractive (pigments) colour mixing.

 

How can we be sure when we see a colour it is a pure colour?

By using a spectrometer to measure it, I guess.

I've spent a few entertaining hours looking at colour and light on the web, its not an easy subject by any means. Like a lot of science it gets tricky when you probe. I supose its the nature of the beast.

I discovered the different colours travel at different speeds in glass. A most curious fact and yet in a vaccum they all travel at equal speeds.