Most people see pink, red and two shades of green. Look closer and you will see there is only one shade of green and one of red in the picture.

The colour you see depends on which other colours are surrounding it.        Here, green next to red looks much darker than green next to white.

This effect seems to be due to ‘lateral inhibition’. This is the mechanism that your eye and brain uses to make edges, outlines and colours more distinct. About 90% of this effect is due to cells in your eye.

This effect has been used for thousands of years by carpet makers to make apparently multi-coloured carpets from just a handful of different wools.