Well, black people can get a tan. I dated this black woman who was a lifeguard for a few months when I did a gap year in college. She had to wear one of those one piece lifeguard suits, and she was out in the sun a lot. Her skin tone wasn't light, I'd say she's a bit darker than Storm in this game. But when she would show off her midriff for instance, you can see that it was much lighter then say her arms or face that were constantly getting exposed to the sun.
So yeah, Storm can get a tan, she'd just be darker. And there are some black men and women who tan to have a darker skin tone.