to answer types...
purple- Magic
blue- Hit
red- Slash
yellow- Pierce
grey (whaleship only)- Generic (as in no type at all)
but yeah, when you have very few 6* girls, any is welcome so level the crap out of them and be sure to get 2nd affection. It's rare if a 5* can come close to the stats of a 6* and even rarer if their abilities can come close.... the gap in power and usefulness is usually noticeable.
at least the game has enough leeway to let you progress quite well with just the event 5* girls. (some concept missions can destroy you though), but savor every 6* you get cause they're kinda difficult to have without buying them