Well Rarity would make an adorable color match sort of pair, making the foals friggin' adorable and most likely gorgeous, but she HATES getting dirty, and can't fly.
Rainbow Dash is SUPER competitive, which could lead to problems in the relationship. All foals look adorable with rainbow dash with her rainbow main and light blue coloring. Also, they are into the same type of things; speed, winning, and flying. The only problem is competition here.
Applejack... The coloring match would be kind of weird, but she is a very nice pony, strong and fast, I think they would make the best match that would actually work if we go by the real standards of their behavior in the show, and how they would really react.