I also noticed this "anomaly", but after a bit of thought it did not seem particularly strange. To my way of thinking, people who use the "-our" ending typically tend to use hyphens when using the "re-" prefix and would be averse to dropping the said hyphens. In addition many dictionaries do not list every single instance of when the "re-" prefix can be used. For either or both of these reasons, or some other, Alan's dictionary sources did not contain recolour when recolor was included. Recolour does now appear in online dictionaries, including Wiktionary, Collins and the Oxford Dictionaries, so perhaps Alan will see fit to add it to the Chihuahua lexicon.