We have designed and synthesized various diphenylether diazonium salts to produce a new fixable red-color-forming diazonium salt. The 4′-n-butyl-4-diazodiphenylether hexafluorophosphate (DED-PF6) obtained forms a vivid red color azo-dye, can be decomposed by 310-nm UV light, and shows good thermal stability. Combined with another commercially available diazonium salt, fixable black-red dual-color thermal recording (DCTR) was achieved. In red printing, the DCTR media allows additional recording and, when necessary, can be fixed by 310-nm UV light exposure. The DCTR is composed of only a single recording layer, which simplifies the manufacturing process compared to multi-layered recording media. DCTR also simplifies the printer mechanism, because it is not necessary to change the temperature of the thermal head between black printing and red printing; simply adding a fixing lamp to a conventional thermal printer is enough for DCTR.