How can you predict which metal cations will form colored precipitates when Na2CO3 is added?

I've been trying to figure this out for 3 days and I have no clue how to. We're working with various ions such as MgSO4 and FeCl3. I know if a substance has its d-orbital partially filled it is usually colored, but how can I predict whether they'll be colored after I add Na2CO3 and what color will they have if any. Also what experiment can I do to figure this out?