"in colour" vs "in colours"

Neither seems possible to me from your brief description. Do you have a sentence?

for example: dogs dream in black and white, people dream in colour/in colours. I believe it's an expression, it's sometimes used when saying something is colourful, but I've seen both versions. I don't know if they're both correct, only one of them is or if they're both wrong.

Ah, yes. The usual phrase is 'in color': I dream in color; the movie was in color (not in black and white). There is nothing wrong with 'in colors'; it is just not so usual.