Question

What will be the colour of a shadow formed in red light?

Answer 1

The shadow is the place where the light doesn't go. So it doesn't matter what colorthe light is where it does go, and the shadow is usually described as "black".

Answer 2

Not all shadows are black.
When lights of different colors shine on the same spot on a white surface, the light reflecting from that spot to your eyes is called an additive mixture because it is the sum of all the light. We can learn about human color perception by using colored lights to make additive color mixtures.