As the friction material wears, it ends up staying in suspension in the fluid making the color dark. It isn't getting dark due to it overheating.

I've pulled apart a few .6 trans that were in the 164 K mile range , had original filter ( date code was prior to build date ) and the amount of material in the pan was near zero. You would have thought the trans has 30 K miles based on what was in the pan.