azure lintel
Instead of actually adding a whole new element to the game (so that the powers are fair) you make a recolor of the flame, change the effects on it, and/or shape. Same with projectile fire balls
Example Recolor: Blue fire.
Example Effects: White(default color) plasma beam
Example of Recolor and Effects: Shoot Purple ice darts.
analog canyon
that would be really cool but sadly devs said it would be too confusing (because Rage fireballs are already blue) 
I think it was somewhere in #questions-for-devs-old but quite a while ago
dull vigil
Honestly i think they're exaggerating the potential confusion. Because while yes rage already has blue and a more saturated red as staples, we could honestly get radioactive green as a color option, black flames, black and red flames (Balerion reference) and other colors that aren't just Blue or a deeper shade of red. We got a whole damn rainbow to work with, why limit ourselves to blue, red and deeper red?
quiet ruin
Rage fireballs can stay blue and berserk ones can stay red. The only person who actually sees them is the player who shot them.
Enemies only see red indicator no matter what kind of fire ball it is.
So I am pro-coloring-firebreath/fireballs xD