A direct thermal printing medium is described that incorporates two color-forming layers separated by a thermally-insulating layer. Any combination of the two colors may be obtained by heating with a conventional thermal print head. Independent addressing of each color is achieved by controlling the time of heating and the power supplied to the heating elements of the print head. In combination with a third color-forming layer that is printed from the reverse side of the medium, full-color imaging with photographic quality is possible using two passes with a single thermal print head or a single pass with two print heads. A novel thermal color-forming process based upon phase change of colorless crystals provides images of excellent quality and stability.