The highlighted part please

The abnormal hemoglobin causes red blood cells to become rigid, sticky and misshapen. Both mother and father must pass the defective form of the gene for a child to be affected. If only one parent passes the sickle cell gene to the child, that child will have the sickle cell trait.Source Mayo Clinic

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.