The proteins in a biological sample that is sought to be analyzed for its protein composition by an electrophoretic or chromatographic procedure are coupled to a dye in an unusually efficient manner by combining the sample with a solid dry composition containing the dye, a buffering agent, and in preferred embodiments, a denaturing agent as well. The solid and dry form of the composition prevents the dye from deteriorating or decomposing, and the combination of components in the composition allows the dye to couple to the proteins in a relatively uniform manner with no overstaining of the protein when the composition and the sample are heated together and held at an elevated temperature for a short period of time.