User blog comment:Omega natsu2/Chapter 413: The Book of E.N.D., Predictions./@comment-23.117.125.45-20141219030037/@comment-7418318-20141221232242

END is a Demon, so his flames would probably come from a curse rather than magic and thus are different than regular magical flames or Dragon Slayer flames at least as much (probably more so) than God Slayer flames.

But I don't think your statement is true in the first place. Sure we've seen differently coloured flames have different effects, but we've also seen regular (non-magical) fire, magical fire, Dragon Slayer fire, Atlas Flame's Hellfire and whatever kind of fire is produced by Mirajane's Sitri-form all share the same looks, so different effects can still come with the same colour.

And then we also have the opposite with Ice-Make, where the four users each have their distinct coloured ice even though the effects are the same.