There is no difference in meaning — only in country. Color is the American spelling ("the color of the sky"); colour is standard in British, Irish, Australian, New Zealand, South African and Canadian English ("the colour of the sky"). Pick one according to your audience and use it consistently.
The core difference
This is a spelling convention, not a distinction in sense. Colour follows the older -our pattern; American spelling dropped the u in a group of words including honour/honor, favour/favor, neighbour/neighbor and labour/labor. Both spellings are fully correct within their own conventions, and neither is more formal or more educated than the other.
Which to use
- Color: United States. Also generally the Philippines.
- Colour: United Kingdom, Ireland, Australia, New Zealand, South Africa, India, and Canada.
Canadian English is worth flagging because it mixes conventions elsewhere, but on this word it firmly keeps the u.
The test that settles it
Ask who is reading. If your reader is American, drop the u — everywhere, not just in this one word, or the page will look inconsistent. If your reader is anywhere else in the English-speaking world, keep it.
The rule extends predictably to the whole family: colourful/colorful, coloured/colored, colourless/colorless, colouring/coloring, discolour/discolor, watercolour/watercolor.
The exception that trips people up
One derivative loses the u in both varieties: coloration. "The bird's coloration" is standard in British English too, though colouration appears often enough to be listed as a variant in some dictionaries. The same pattern affects honour/honorary and vigour/vigorous — when a Latin-derived suffix is added, the u tends to disappear.
Code, brands and proper names never change
Technical contexts use American spelling regardless of where you are. CSS properties are color, background-color and border-color; HTML attributes and most programming libraries follow suit. A British developer writes color: #333 in the stylesheet and "choose a colour" in the interface text, and that is correct in both places.
Proper nouns keep their own spelling too. The Color Purple, Color Me Badd and any company registered as Color Ltd stay as they are, even in British prose. Quotations should be reproduced with their original spelling.
Consistency matters more than choice
Editors care far less about which convention you pick than about whether you stick to it. Mixing colour with favorite in the same document is the mistake that reads as careless. Set your spellchecker's language to match your audience and let it enforce the whole -our family at once.
Frequently Asked Questions
Is color or colour correct?
Both are. Color is correct American spelling, colour is correct in Britain, Ireland, Australia, New Zealand, South Africa and Canada. Neither is an error in its own region.
Which spelling does Canada use?
Colour. Canadian English keeps the u in this word and in neighbour, favour and honour, even though it follows American practice on some other spellings.
Should I write color or colour in CSS?
Always color. CSS properties use American spelling and colour simply will not work. Your interface text can still say colour if you write British English.