My hair is yours..
dark
darker
dark than
darker than

Darker than

Explanation:

My hair is dark yours. (Incorrect. Grammar does not make sense. You don't use this when comparing one thing to another)

My hair is darker yours. (Incorrect. Grammar is incorrect)

My hair is dark than yours (Incorrect. You still don't use Dark when comparing)

My hair is Darker than yours (Correct! The grammar is correct and the correct comparative form)

When comparing one thing to another you add -er.

When you are comparing 3 or more things you add -est.

(Sherry is the greatest singer in the quoir)

(Yui has larger eyes than Terri)

it the character ahdfg gsytriw   errtywer erwqtydfgb gwty