So, whenever Teddy Lupin's emotions change, so would his hair. If he's angry, his hair would turn red. If he's happy, it'd turn a gold or yellow. He'd change it back to whatever he wants after he realizes, but it would be just a thing that happens naturally. I also like to think that Victoire found out Teddy liked her when his hair would turn pink whenever they looked at each other. This also made it easy for Andromeda to understand Teddy better.