In my experience, having it as hot as possible is the only way to dye some of the harder plastics. Hot enough that you do risk warping the plastic if you leave it in too long. So... Don't leave it in too long.
I usually get the pot boiling, then put my parts in a wire basket. While the pot is still boiling, I dip the basket with the parts in for about 10 seconds (not much longer!) then pull it out and dip it into a pot of cold water. And then repeat, repeat, repeat until you're happy with the color that the plastic has soaked up.
Or until the dye has started to break down and starts to become a film on the surface of the water. When that happens, stop and go back to the beginning. With new dye and a new pot of water. Or else the film on the surface will stick to the plastic, giving you blotches or specks rather than an even color.