Submersible heaters for aquariums don't reach the temperature required for colour processing (38 °C) because fishes would die. If you develop colour that means you'll have to "modify" your heater (break it so that the knob turns freely beyond the maximum temperature).
I think for your case the best is one of those food-warmers with a thermostat. I assume you only need one tray with really correct temperature. You will probably be able to find some in a second-hand store or used on the internet.
Something like this:
make sure there is a thermostat.
No water, no light (or easily maskable) and no great expense either.