What is the most common colour in the universe?
not really possible to tell because we don't know enough of the universe
in actuality, the "color" of light you would see most would be all of them, or white. light wave frequencies become distinguishable when light is refracted through a material, like water for example. Since in the universe, most light is never refracted, it would remain the same "white" our brain sees all the colors meshed together as.
I would say it depends on what you mean by colour. Some would say black and white are not colours, but in printing black is one of the colours used. So if black is accepted as a colour, I would definitely say that it's the most prevalent colour in the whole universe. Look at the night sky.