Aryl grignard do change colors. mesityl grignard is somewhat greenish. I wouldn't be too put off by the color.
by digrignard do you mean a dialkylmagnesium compound, R2Mg? That is probably not likely to happen. Magnesium undergoes oxidative addition into the R-X bond, which forms the RMgX compound and oxidizes Mg from 0 to +2. For a second alkyl group to add, the RMgX would have to either undergo another oxidative add'n (which would push Mg to +4... unlikely). alternatively, two grignard reagents could transmetallate: RMgX + RMgX --> R2Mg + MgX2. I don't know how likely that would be, but I doubt it would occur to a large extent... maybe with enough heat, but I still don't see it happening...
wait. upon closer reading, you say p-bromobenzene. what is the bromine para to? do you mean p-DIbromobenzene? That changes the question completely. I don't have any experience with grignard SMs with 2 reactive sites. I'll defer if that's what you're actually talking about.