Let's plus ultra!

so when he’s Midoriya’s age his hair is sunflower. Then when he’s in his prime we see him with the TUFTS. Then when Midoriya meets him for the first time, he’s got the TUFTS in big mode and then when he goes Smollmight his hair returns to sunflower.

methinks that his hair always had TUFT mode and sunflower mode and it depends on whether OfA is active or not since he almost certainly would have learnt to have the power flowing through him continuously when activating it like Izuku. I reckon that before his big battle with AfO that left him sans a lung and smol his hair would have gone from TUFT to sunflower when he “powers down” so to speak. Also his big mode voice might just be something he does for effect rather than a natural side effect of big mode..

Edit: I HAVE ADDED AN EXAMPLE!!!

This one, I'm proud of still.

The former number one hero of japan......

The man himself.....