Some dyes and fabrics, especially colored shirts, react to heat and shift in color. A lighter shade of the color is typical after pressing when this is an issue. Sometimes, the color and garment overall will recover after a few hours. Setting the garment aside and looking at it later in the day in a different light may reveal the issue was only temporary. If the color shift remains, you must find the highest temperature that does not do this by lowering the temperature setting in 25-degree increments. Raise the time setting to compensate for this. Oftentimes, that particular shade/dye/shirt simply will not work with the transfer settings/recipe/requirements and a different shirt brand and dye must be used.