British English and American English. Both are accepted.
 -our, -or
Most words ending in an unstressed -our in British English (e.g. colour, flavour, harbour, honour, humour, labour, neighbour, rumour) end in -or in American English (cf. color, flavor, honor, harbor, neighbor, rumor, labor, humor). Wherever the vowel is unreduced in pronunciation, this does not occur: e.g. contour, velour, paramour and troubadour are spelled thus the same everywhere.
Most words of this category derive from Latin non-agent nouns having nominative -or; the first such borrowings into English were from early Old French and the ending was -or or -ur. After the Norman Conquest, the ending became -our in Anglo-French in an attempt to represent the Old French pronunciation of words ending in -or, though color has been used occasionally in English since the 15th century. The -our ending was not only retained in English borrowings from Anglo-French, but also applied to earlier French borrowings. After the Renaissance, some such borrowings from Latin were taken up with their original -or ending; many words once ending in -our (for example, chancellour and governour) now end in -or everywhere. 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 of Latin origin (e.g. color) and -our for French loans; but in many cases the etymology was not completely clear, and therefore some scholars advocated -or only and others -our only.
Webster's 1828 dictionary featured only -or and is generally given much of the credit for the adoption of this form in the United States. By contrast, Dr Johnson's 1755 dictionary used the -our spelling for all words still so spelt in Britain, and others where the u has since been dropped: ambassadour, emperour, governour, perturbatour; inferiour, superiour; errour, horrour, mirrour, tenour, terrour, tremour. Johnson, unlike Webster, was not an advocate of spelling reform, but selected the version best-derived, as he saw it, from among the variations in his sources: he preferred French over Latin spellings because, as he put it, "the French generally supplied us". Those English speakers who began to move across the Atlantic would have taken these habits with them and H L Mencken makes the point that, "honor appears in the Declaration of Independence, but it seems to have got there rather by accident than by design. In Jefferson's original draft it is spelt honour." Examples such as color, flavor, behavior, harbor and neighbor scarcely appear in the Old Bailey's court records from the 17th and 18th century, whereas examples of their -our counterparts are numbered in thousands. One notable exception is honor. Honor and honour were equally frequent until the 17th century; Honor still is, in the UK, the normal spelling as a person's name.