^ Touhou is famously inconsistent with skin hues (see 'Tan Cirno' and 'Zombie Reimu' memes for more), but there's no canon authority on such details, and ZUN would basically say that you're free to imagine what you'd want.
Logically speaking, Reimu does spend a large amount of her day sitting on a veranda or sweeping the shrine grounds under direct sunlight, while Marisa operates out of a dense and shady forest, so it wouldn't be too strange for one of them to have more of a tan.