Which one is correct?
Actually, they are both correct!
Some English words have both British and American spellings. Colour is the British spelling, while color is the American spelling. Which one you see usually depends on where the writer comes from. It is okay to use either spelling.
♦ Some other words that have both British and American spellings:
British spellings: favourite, organise, centre, theatre, jewellery, judgement, defence, catalogue, arguement
American spellings: favorite, organize, center, theater, jewelry, judgment, defense, catalog, argument