Question

what is polymorphism? is color of skin a polymorphic trait?

Answer 1

as its name suggests. a variant of trait. by definition more abundant than 1% of population. ABO group and hair color (and skin color) are all classical examples.
the definition does not coney a negative connotation as a mutation does

Answer 2

it is variation in gene ,chromosome or dna sequence resulting in variations in phenotypes...it is essential for maintaining variability in population