The present invention is directed to the use of a mixture of non-light sensitive organic silver salts in a color photothermographic system comprising a blocked developing agent for color imaging. At least one of the organic silver salts is the sole or primary silver donor during thermal development and at least one other organic silver salt, present at levels in the range of 30,000 to 60,000 mg/mol of silver halide, effectively inhibits fog during thermal development of the photothermographic element. In one embodiment, and the system comprises a mixture of at least two organic silver salts, wherein the first organic silver salts exhibits a cLogP of 0.1 to 10 and a Ksp of 7 to 14 and wherein the second organic silver salt exhibits a cLogP of 0.1 to 10 and a Ksp of 14 to 21.