we could see various types of fish which are of different sizes and colours. The sentence ok?

Question

Answer

I think you need "...which were of..." because you're using "could" (past), not "can".

"we could see various types of fish which were of different sizes and colours."

If you want to make it less wordy, use "we could see various types of fish, of different sizes and colours."  Don't forget the comma! :)

Answer

No problem IMO.