Question

suggest two ways by which we can get light of a single colour to fall on the screen after passing through a prism

Answer 1

you can isolate by passing the light through a coloured filter.
the other way could be cutting a hole through the paper to only allow the range of colour you want through.

Answer 2

I believe the light exiting the prism will be polarized -- I know that rainbows are polarized. Maybe you can use polarizers to selectively allow the wavelengths you want.