"The color of your forehead is white."
Translation:Culoarea frunții tale este alb.
possessive pronoun feminine singular ta also looks like plural in genetive case: tale - just like the unarticulate nouns in singular (fată » fete). And like unarticulate nouns, masculine does not change: Culoarea umărului tău... (not like pl. "tăi")
alb is not meant to be an adjective to culoarea, but rather a noun for the colour white. Culoarea e albă când e uscată would probably mean that some paint appears white when it is dry (adjectives)