DPD reagents are a colorimetric method based on N-Diethyl-p-phenylendiamine that, when in contact with a sample of water, react, making it turn magenta in proportion to the concentration of chlorine present in the water. DPD reagents are white in color and have a crystalline appearance.
When a reagent (tablet or powder) is not white and has a gray/brown color it should not be used. Liquid reagents on the other hand are usually colorless, when they are brown they should not be used. To avoid erroneous results, avoid using reagents in poor condition.