Colonist's claws and skin must always be darker than the fur they're touching. They also should be either similar to the base coat near them in saturation and coloration or natural in shade. Remember that fur colour is dictated by skin colour in most cases, so a hairless colonist with white spots will have areas of darker skin and areas of lighter skin.
Horns and beaks can be darker or lighter than the markings around them. They can be ivory, black, white, or similar natural colours. Beaks can be orange or warm yellow, like real world birds. They can also be affected by markings on the body, even if they are near the tail or affected by artifice. If you have artifice, you may pick just the horn/beak as your affected body part/marking if you would like!