Disclosed is a colorant treated ion exchange resin comprising at least 15% of exchangeable groups comprising at least one of an ion, a Lewis acid, or a Lewis base resulting from a colorant having a pK.sub.a or pK.sub.b of greater than 5 in an aqueous solution at 25.degree. C., based on the total number of exchangeable groups. Also disclosed are heat transfer systems, assemblies, fuel cell systems and methods of maintaining a conductivity of less than 200 .mu.S/cm in a heat transfer fluid that employ the disclosed colorant treated ion exchange resins. Finally, a method of making the disclosed colorant treated ion exchange resins is provided.