The old Frankford Arsenal case cleaning solution recommendation was 5% citric acid. The discoloration you are seeing is oxidation by the air. Being warm greatly accelerates that oxidation. If you want to use heat, I would recommend flushing the acid solution out with cold water followed by a little distilled water to eliminate watermarks left during drying.
Citric acid micro-etches the brass surface. This means it can oxidize easily. If it happens slowly, that's not an issue as the oxide layers actually offer the brass some protection. But that doesn't help the color and the Hornady ultra-sonic cleaning formula uses a little bit of diethanolamine, an anti-oxidant, to help cut that down. The fellows mixing a one-step car wash and wax also get surface protection from oxidation via the wax coating. However, tumbling with stainless pins, I prefer to use the citric acid solution alone and stop halfway through the cleaning and exchange the acid solution for the washing and waxing solution. This is to keep the two products from mixing or messing with the pH of the acid solution.