Purple End Fire doesnt work when True Ending: Ender Dragon Overhaul is installed. i assume its because the mod uses soul fire instead of regular fire.

Yeah that is the case and I won't change that

why not? you replace regular fire with purple fire. why not also replace soul fire with purple fire? is there a technical reason behind it?

The reason behind this is that I don't want to change the soul fire. The soul sand Valley overhaul option changes regular fire and this as well.