Electrons have no colour.
What is you see is the effect that electrons have on the coating of the cathode ray tube (CRT), in your case (probably) zinc sulphide
The electrons that hit the zinc sulphide bring it into a higher energetic state, and when it falls back, it emits light, in this case green.
If you use other so-called phosphors in stead of
Something similar happens in luminescent tubes, where the invisible UV light is 'translated' into different colours of visible light to give an altogether white-light effect.