TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The colorant comprises at least one of the following chromophores: anthraquinone, triphenylmethane, diphenylmethane, azo containing compounds, disazo containing compounds, trisazo containing compounds, diazo containing compounds, xanthene, acridine, indene, phthalocyanine, azaannulene, nitroso, nitro, diarylmethane, triarylmethane, methine, indamine, azine, oxazine, thiazine, quinoline, indigoid, indophenol, lactone, aminoketone, hydroxyketone, stilbene, thiazole, at least one conjugated aromatic group, at least one conjugated heterocyclic group and/or at least one conjugated C-C double bond (preferably anthraquinone, triphenylmethane, diphenylmethane, azo containing compounds, disazo containing compounds, trisazo containing compounds and/or diazo containing compounds). The colorant comprises the reaction product of a chromophore and a non-conductive 1-3C alkoxy radical. The colored heat transfer fluid comprises a non-conductive colorant (0.0001 - 0.2, preferably 0.0005 - 0.1, especially 0.0005 - 0.05 wt.%), and has a conductivity of not greater than 10 (preferably less than 5, especially 0.02 - 5, particularly 0.05 - 1) muS/cm; and further comprises an alcohol. The alcohol is at least one of methanol, ethanol, propanol, butanol, furfurol, ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, dipropylene glycol, butylene glycol, glycrol, monoethylether of glycerol, dimethyl ether of glycerol, 1,2,6-hexanetriol, trimethylolpropane and/or methoxyethanol. The colorant in the ion exchange resin and the non-conductive colorant in the colored heat transfer fluid are the same. The colorant is of formula R(Ak((B)nR1)m)x (I). In the method, the colored heat transfer fluid has a color that is visible to the human eye.R=organic chromophore selected from anthraquinone, triphenylmethane, diphenylmethane, azo containing compounds, disazo containing compounds, trisazo containing compounds, diazo containing compounds, xanthene, acridine, indene, phthalocyanine, azaannulene, nitroso, nitro, diarylmethane, triarylmethane, methine, indamine, azine, oxazine, thiazine, quinoline, indigoid, indophenol, lactone, aminoketone, hydroxyketone, stilbene, thiazole, at least conjugated aromatic groups, at least one conjugated heterocyclic groups and/or at least one conjugated C-C double bonds;A=linking moiety in the chromophore and is selected from O, N or S;k=0 or 1;B=1-8C alkyleneoxy or 1-8C alkoxy;n=1 - 100;m=1 or 2;x=1 - 5;R1=H, 1-6C alkyl or 1-6C alkoxy groups. EXAMPLE - No relevant example given.