Note that having a red colour implies that it absorbs all but a low wavelength of energy - while having a green colour would imply that it absorbs all but a higher wavelength of energy. This could be significant.
It could just be a random animation thing and they decided, now that they have a higher budget a few years later, that red with purple lightning looked cooler and red was naturally the most evil color for the gems. However, theories are more fun. :3