Answer: Rosa only looks feminine. In this case it's really a masculine adjective.
It's true that the vast majority of adjectives ending in -a are a feminine form of an adjective ending in -o or, less often, some other ending. But there are a few adjectives that end in -a in their "natural" state (i.e., in their singular, masculine form), and they don't change with the gender of the noun they describe.
As you might have guessed, rosa is one of those. We say la flor rosa for "the pink flower," but we also say el carro rosa for "the pink car."
Of the adjectives that naturally end in -a, and there are very few, most of them are (like rosa) derived from nouns. They include homicida (homicidal), hipócrita (hypocritical), and suicida (suicidal). Some adjectives of nationality, such as croata (Croat), also end in -a and are used with both males and females.
Probably the most important adjective that naturally ends in -a is cada, meaning "each." E.g., cada hombre debe salir, each man ought to leave.
If you're wondering whether an adjective ending in -a is feminine, look it up in a dictionary. Dictionary listings for nouns and adjectives are in the singular masculine form.
