Tanning beds to deepen your skintone (with different levels/durations of the treatment giving you varying shades of darkeness) are an easy choice here, but there are room for other creative things. If a darker skinned character wants to lighten their skintone, perhaps nanogenic treatment or some other machine can come into play to accomplish that.
This would also play into the disguise system nicely - it'd be hard to recognize JoeBaka who you know to be white as Vanilla Silk, who is now rocking a nice tan.
Thoughts?