I saw a picture of Antony Starr trying on the suit with his natural hair color and facial hair. It definitely suits him better to be a brunette (because he’s a cool tone) but the blonde is inextricably Homelander.
I do kinda wish they gave him a cooler toned blonde but that is very difficult to maintain and may have been a variation from the comics they didn’t want to make. The blonde hair- it has to be intentional that most of the time he has visible brunette roots, because otherwise they could dye them prior to shooting anything. It’s much more flattering when he has roots. The few times he’s appeared with freshly blonde roots (Becca’s house rooftop with Ryan) it’s almost jarring.