I'd say the normal shades of White, Black and Gray, but also metallic colors like Silver, Copper, Gold and so on.

I think a unicorn is mostly white,but there are a few black ones roaming around that are just harder to find.

Pure white but the horn is golden the white is the white of a freshwater pearl

Cream with an iridescent mane.

Putting aside what they 'should' be, the ones I use in my writing range from black to white - and any shade of grey inbetween. Their manes, likewise, can be any shade of grey, though does not necessarily have to match the rest of them.

silver, they absolutely gotta be silver

I think their body has to be white to show purity (ya know, cause they only show themselves to virgins, right?) but their maine and tail could be a light shade of whatever color..

Depends what genre for example if it's for childrens genre it could be any colour for fantasy it's usually white and sometimes black (more of a modern take on it) and can they can look like anything depending what the story behind the picture is determines what colour it can be and still fit in. For example you wouldn't expect a rainbow coloured pony in a very gothic scene of dark thick forest filled with a sea of fog (not very scary). Depends on what effect your looking for.

It's usually white in all the stories. I really like your black unicorn artwork. If I were to do a unicorn art, I'd probably make it blue or white body with blue mane and tail.