Not all of the light would have been wasted on the wall. If your wall is painted green, then the 'rest of the rainbow' (red, orange, yellow, blue, violet wavelengths) would be absorbed and converted into heat. Paint is quite rough on a microscopic level, and the green light reflected would be scattered in every direction.

Things that have a colour do so because they reflect those frequencies. Mirrors reflect pretty much all frequencies of visible light with very little scattering - that's the definition of the word, really.

If you had a black feature wall behind your lamp, such that very little was reflected off it into the rest of the room, then with a mirror there would be about twice the photons illuminating the room. If your wall was pure brilliant white, much less of a difference. Your eyes don't perceive 'twice the photons' as 'twice as bright' - they scale from absorbing thousands a second when fully dark-adjusted at night, to trillions per second at midday - but you might find it a bit easier to eg. read a book elsewhere in the room.

Light output from the lamp doesn't change, but depending on the colours of things in your room, the light output that is useful for seeing might do.