Color factor for squark-quark-antiquark vertex

Question

I am trying to do a tree level calculation of cross section in a process that involves sbottom exchange. There is a $ \widetilde{b} q \bar{q} $ vertex, where $q$ and $\bar q$ are quark and anti-quark of the same flavor. I have following questions:

1. How is the color conserved in such vertices? The problem is that (s)quarks carry only one unit of color.

2. What will be the color factor from this vertex if written explicitly? (Just like Griffiths does in his book)

PS: My knowledge of QFT, QCD, and SUSY is very limited, almost none. I got the vertex factors and propagators from the superpotential for MSSM without R-parity using the method outlined in Griffiths' book. My answer is correct upto a color factor, hence I know THAT is the source of the problem.

This post imported from StackExchange Physics at 2014-05-04 11:30 (UCT), posted by SE-user negligible_singularity