I don’t want to be pedantic about this, it looks as though this is a British/US difference. For those who like to get the full story on these things there is more from Wik.
-our, -or
Most words ending in an unstressed -our in British English (e.g., colour, flavour, honour, neighbour, rumour, labour) end in -or in American English (e.g., color, flavor, honor, neighbor, rumor, labor). Wherever the vowel is unreduced in pronunciation, this does not occur: contour, velour, paramour, troubadour, are spelt 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.[29] After the Norman Conquest, the termination became -our in Anglo-French in an attempt to represent the Old French pronunciation of words ending in -or[30], though color has been used occasionally in English since the fifteenth century.[31] The -our ending was not only retained in English borrowings from Anglo-French, but also applied to earlier French borrowings.[29] After the Renaissance, some such borrowings from Latin were taken up with their original -or termination; 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[31]) 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.[32
There is more than that too!
