If due to exposure to magical or environmental effects you were born with an unusual eye or hair color that is outside the normal range for a human, would you be unable to use a second dose to return to your previous appearance after using the first one to aid in a disguise?
If two PCs are identical twins and one got a tan from a sunny vacation and the other could not join in, would the paler twin be able to match the tan with a serum of appearance change? Would this violate the restriction against taking on the exact appearance of another creature.