Answer:
Constitutive is natural skin color and facultative is the darkening due to UV or other factors.
Step-by-step explanation:
Constitutive skin color is the natural color of the skin, and that color has nothing to do with any controllable/external factor and is determined in your genes, while Facultative is the result of ultraviolet light or other environmental factors which causes a change in melanin in the skin therefor making it darker