Why does British English include a u in colour?
American and British English spelling differences
Most words of this kind came from Latin, where the ending was spelled -or. They were first adopted into English from early Old French, and the ending was spelled -or or -ur.[9] After the Norman conquest of England, the ending became -our to match the Old French spelling.[10] The -our ending was not only used in new English borrowings, but was also applied to the earlier borrowings that had used -or.[9] However, -or was still sometimes found,[11] and the first three folios of Shakespeare's plays used both spellings before they were standardised to -our in the Fourth Folio of 1685.[4] After the Renaissance, new borrowings from Latin were taken up with their original -or ending and many words once ending in -our (for example, chancellour and governour) went back to -or. Many words of the -our/or group do not have a Latin counterpart; for example, armo(u)r, behavio(u)r, harbo(u)r, neighbo(u)r; also arbo(u)r, meaning "shelter", though senses "tree" and "tool" are always arbor, a false cognate of the other word. Some 16th- and early 17th-century British scholars indeed insisted that -or be used for words from Latin (e.g., color)[11] and -our for French loans; but in many cases the etymology was not clear, and therefore some scholars advocated -or only and others -our only.[12]
English orthography
It is, however, not the shortage of letters which makes English spelling irregular. Its irregularities are caused mainly by the use of many different spellings for some of its sounds, such as the sounds /uː/, /iː/ and /oʊ/ ("too, "true, "shoe, "flew, "through; "sleeve", "leave", even", "seize", "siege"; "stole, "coal", "bowl", "roll", old", "mould"), and the use of identical sequences for spelling different sounds (over", oven", "move").