The present invention pertains to a reversibly thermochromic system comprising a substituted or an unsubstituted 6,11-dihydroxy-naphthacene-5,12-dione and a base and to a reversibly thermochromic composition comprising the reversibly thermochromic system and a carrier material and to novel substituted 6,11-dihydroxy-naphthacene-5,12-diones.
