I have always attributed the colored speech bubbles of the certain angles, demons, the 7 and sometimes their minions; as bits of their Power leaking into their voice, since the art is so concerned with the voice of its wielders.

I think the fact that his speachbubble is vanilla may just mean he has better control over what he says.

That when he looses his cool talking to BIG CHUNGUS in https://killsixbilliondemons.com/com...-thrones-4-29/ and the text turns hot pink seems to validate the theory.