It amuses Hawks to think Enji probably doesn't know his fire has so many different shades. The man has more than forty years using his quirk and never learned such a nice detail about it.
Hawks of course had always noticed it. When all he had was a tiny TV with a blurred image and not so great color pallet, he could still tell his favourite hero's flames had different colours from time to time. His little child brain could figure it out the shades depended on the intensity of the fight, but not exactly why.
After meeting the man in person, he soon realizes they all mean emotions.
Flaming orange for anger. The most common one. The one coloring his beard and his flames when a villain pisses him off or hurts civilians. Enji's trademark colour when he gets irritated by one of Hawks innocent jokes is slightly redder, but still orange. But that's how Hawks knows he's not actually furious with him, just super annoyed. Which maybe sound the same, but trust him, it's a shade different.
Pink for pride. Enji probably would think he looks silly with pink fire, but Hawks thinks it's cool. It's subtle, just little sparks here and there when he uses his hell flame for cool number one purposes. Hawks can tell when he's full of himself, when he knows he looks good and is strong as hell. He loves the arrogant bastard.
Yellow, fatigue. Hawks hates seeing it and having his heart skip a beat in worry, because the number one hero must be pretty tired to let it be seen through his flames. But Hawks has blind fate on the man, so he never worries too much. He believes Endeavor can make through anything, no matter how exhausted he is, no matter how many times he falls, Endeavor-san's strength is that he always get up.
Enji's flames get faded as he gets sad, and it breaks Hawks heart whenever he sees it. The colours are almost the same, but the saturation is all wrong, his fire gets pale, and it's really obvious how sorrowful the man is. Except Hawks has never seen anyone mention it, not even at fan forums, so he keeps that knowledge to himself. Just remembers to buy some sweets and drop by at his office when it happens.
Lately though, something strange is happening. When they are together, specially just the two of them, he starts to see a different colour.
He can swear to god he had never seen that one before. He goes through his collection of Endeavor videos just to make sure, but it isn't there. It's new, dammit, he's the Endeavor fanboy, he can't be mistaken.
And the new mysterious color it's the same red as his wings. Of course, he fucking notices because he was born with them, and it looks so identical it's impossible to mistake. Hawks used to despise that color. Bloody scarlet, as he used to call it because his wings were only good to do dirty deeds.
It doesn't look hideous on Enji's flames though.
Hawks is actually anxious to know what sort of emotion it means. He kinda knows it, it's not even that hard to guess, and he's not a moron. But still, it's something he wants to discover for himself. And maybe, just maybe, hear the man say it.