Some indicator dyes can exist as either a colorless or a colored tautomer, depending upon the environment. We have found examples of fluoran dyes (fluorescein, rhodol, and rhodamine) for which there exist crystalline forms of the colorless tautomer, but which in other crystalline habits or in the glassy state exhibit substantial absorption in the visible. The melting of colorless particles of these dyes provides a mechanism for direct thermal imaging characterized by high sensitivity for a given level of stability. The optimization of these amorphochromic dyes for direct thermal imaging will be described.