Colour puzzle

These lights - a work by Jiri Thyn in the "Drawing and Photography" show at the Photographers Gallery until 3 July - have coloured filters: red, blue, green (from left to right).

If those lights are red, blue, green ... why is are the colours on the wall so different?

If you (discreetly) put your hand in front of the light, it becomes the colour of the filter - but its shadow is yellow, cyan, or magenta.

It feels - or looks - like magic.