Why the heat?

You will find that all Predators in all the movies see in the infra-red end of the light spectrum when they remove their bio-mask's because this is how they naturally see. The bio-mask sharpens their natural vision while allowing them to see the environment using other point's in the light spectrum, as shown in Predator 2 and the 2 AVP movies.

But in P1 its vision was all red. Same with the Berserker Super Predator.

Yes, their vision was red (and yellow) in the first Predator and Predators because they were in a hot jungle, hence the colors.