Bigger accuses Gus of being a "yellow black bastard" as a way of concealing his own fear. Previous to this, all of the petty crime that Bigger, Gus, and Jack have committed has been against other African Americans. They feel relatively safe in doing this because they feel that white police officers will turn a blink eye to blacks stealing from other blacks.
Now, however, Jack and Gus want to rob Mr. Blum, the white deli owner. Bigger is scared of this idea because he believes the police will not tolerate blacks stealing from whites. But rather than admit this, Bigger explodes in anger at Gus's tardiness, trying to intimidate him and calling him "yellow" as a way to conceal his own fear of robbing Blum and thereby ruining the robbery plan.