Backstory of the spelling of that word:
Latin colōrem (accusative of color) gets inherited by Old French as color /ko'lor/.
Somewhere down the line Old French shifted /o/ to /u/. I believe this shift affected at first stressed vowels, or that the distinction between unstressed /o/ and /u/ was already not a big deal; so there was more pressure to respell the last (stressed) vowel than the first (unstressed) one. So the word gets spelled color, colour, colur.
Anglo-Norman inherited this mess, spelling it mostly as colur. Then Middle English borrows the word, as /ku.'lu:r/~/'ku.lur/. It's oxytone in AN, but English has a tendency to shift the stress to the first vowel, creating the second pronunciation. Spelling as usual for those times is a mess:
- colur - spelled like in Anglo-Norman.
- color - swap the ⟨u⟩ with cosmetic ⟨o⟩. Scribes hated spelling ⟨u⟩ in certain situations, where it would lead to too many vertical lines in a row; that's why you also got come, love, people instead of cume, luve, peuple.
- colour - mirroring an Old French spelling that was more common up south, around Paris.
- coloure - that ⟨e⟩ was likely never pronounced, I think it was there to force reading the previous vowel as long
- coler - probably from some /'ku.lur/ pronunciation already reducing the vowel to */'ku.lər/
- kolour - ⟨c⟩~⟨k⟩ mixing was somewhat common then. And no, KDE did not exist back then, they did no lobby to spell the word with a K for the sake of a program that would only appear centuries later (Kolourpaint).
Eventually as English spelling gets standardised, the word settles down as colour.
Then around 1800, Noah Webster treats this word as if it was directly borrowed from Latin. Since in Latin it's color, he clipped the -u. And his dictionary was popular in USA, recreating the mess, even after it was already fixed.