SO I ACTUALLY POSTED THE SAME THING EARLIER IN AUGUST. My roosters combs were turning purple as well. The friendly posters in the community told me that it’s because of the heat.
When I have a chicken with an extremely large comb like that, it can be normal to occasionally see a dark comb toward the back or tips of the comb. Heat, cold, excitement all can affect comb color.
I would make sure there were several shaded waterers throughout their area, and add cold water twice a day. Plain pans or bowls are fine to use for extra water. Adding ice to the water helps as well. Mine like to cool their legs in a wide flat pig pans of water half full.