Why does British English include a u in colour?
American and British English spelling differences
They were first adopted into English from early Old French, and the ending was spelled -or or -ur
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]