If you're using a texture to emit light, it will emit whatever color light the texture is. If you're using a light entity, I think you use "_color". Like "_color" "1 .5 .7" would have 100% red, 50% green, 70% blue. I can't remember if you need the underscore or not. It may just be "color". Don't go overboard with this, as it just looks bad.