Blue
Ravenous Bugblatter Beast of Traal
So you are spending a feat, and your action, to do d4 daamge to people that hit you. That's quite a bit lower damage then many other damage focused feats -- it's far from WAY TOO POWERFUL when compared to other cases where you need to spend a feat and your action. And it's not even one you can invoke, it only comes around when you are hit, in melee, successfully. It averages 2.5 damage vs. Hellish Rebuke's 11 average, and only works against melee attacks which is not where you are positioning your caster tiefling to be -- sayign they are the same is hyperbole.If it only had to be a fire spell, you could firebolt or whatever every round and (basically) be constantly wreathed in flames dealing 1d4 fire damage to any creature that hits you with a melee attack. THAT is WAY TOO POWERFUL and I can't believe the designers intended that to be the case.
I'd put it on the side of "in the acceptable power curve, but on the low side".