Responses
I got curious after I read your comment and went back a few chapters and looked, comparing earlier fights before The shadow heart and after. And the shadow heart is definitely what caused the change in color.
I think his color change may also correlate with what color his mana is? Because in the beginning it was a lighter blue if I remember correctly and then started to turn much more electric color and now its purple. But then again the change in color might just be the effect of the shadow heart. Just a manga theory tho ╮( ̄▽ ̄)╭
I just notice that MC aura turned purple and is no longer blue and I was wondering when that happened so maybe around the time when he got his shadow heart? Someone pleaseee tell meee t-t