This may be a silly question, but if I have a Doff who starts out as a low level guy (white), will he have a chance to "rank up" to green, blue, and purple if I work with him enough (give him plenty of chances for mission success)? Or do they all stay at the level you get them at originally?

I didn't know if I was "harming" my Doffs' careers by assigning to missions where they weren't really qualified and messing up their chance for promotion.