I don't think it would make a difference in red or blue at night. Red does seem to be the popular choice. I have a purple bulb for only one of my tub's for night time. I wouldn't say he need's more heat at night. Usually, it's ok if the ambient temp drop's like 5 degree's. The uth heat will stay the same thru out the night. He will decide on his own if he's to cool.