I still remember a system that existed for getting used to cold showers. You would use your regular temperature and then make it cold for just a few seconds.
So every minute you would spend a few seconds under the ice-cold (don't think about it) water. You would increase the ratio of the time spent in the cold until at the end of the week most of your shower would be in cold water.
Maybe this works on the same principle... you just associate the warm water with the shower even though it's actually cold.