I can't remember where I read about that effect of heating on toned prints, but I do remember experiencing it once. Use the lowest temperature that will melt the shellac for the shortest time. That seems to be what those do who have not had trouble with color change.

The color of the image may have more to do with particle size, as you will see a difference in color if you use a fine grain developer on the print. Heating may allow particles to join to form larger particles.