In Overwatch, healing is consistently represented by the color yellow. Mercy's beam, Ana's darts, Moira's piss, Soldier's healyboo, etc. Making Lucio's speed aura yellow and healing aura green would be visually confusing
Idk, he's a Brazilian roller-blading sports-loving DJ...
I think most likely scenario is he was green and yellow based on the Brazilian flag, green means "go", and there was only one other color left for "heal", so it's yellow.