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?
Sure. The rule is mostly there to keep you from disguising your species as something really weird.
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.
No, the rule wasn't written with identical twins in mind. "Me but tan" is definitely within the items ability.