The invention relates to a method of preparing a reaction mixture for Polymerase Chain Reaction (PCR) assay and a solution set for PCR. The method comprises providing a sample solution comprising a biological sample to be amplified in said PCR assay and first colorant providing the solution a first color, providing a reagent solution comprising at least one other substance required for performing said assay and second colorant providing the solution a second color different from the first color, and mixing the sample solution and the first reagent solution for providing a mixed solution to be subjected to the PCR process, the mixed solution having, due to said first and second colorants, a third color different from the first and second colors. The invention significantly aids in pipetting PCR assays.