whenever i read weak hero, i always note whether a character's eyebrows match their hair color in my head. it's fun to speculate which weak hero characters naturally were born with their hair color vs. which characters use hair dye, based on whether their eyebrows are black, and what that might say about their personality or background.
using this rationale, it would imply that:
jimmy bae is naturally a strawberry blonde. you could headcanon that his family is mixed and/or he won the genetic lottery when it comes to inheriting recessive traits
donald na is naturally platinum blonde. which is unsurprising, since it's pretty much confirmed by canon that he has albinism
gerard jin was born with green hair. this is the only one that's fully unrealistic to be honest (or: he's the only character who bothers to dye his eyebrows)
rowan im is naturally a redhead, though he has a mix of panels where his eyebrows are fully shaded in and panels where you can see that they're red. either he's another victor of the genetic lottery, or he dyes his eyebrows whenever he feels like going the extra mile
meanwhile, the majority of unnatural hair colors are the result of dye jobs:
teddy jin bleaches his hair, which definitely feels in character for him, since he's established to be the type of kid who cares about his appearance a lot and being trendy
gray yeon might also bleach his hair, which feels distinctly out of character for him, especially if he's being doing it since at least middle school. though, it's hard to tell for gray because his eyebrows are so thin. if anyone can find a panel where gray's eyebrows aren't shaded in, feel free to share with the class
jake ji dyes his hair. however, since he shares his ginger hair color with his older brother kenny, i'm inclined to believe it's actually genetic. though, the potential headcanon that jake looks up to kenny enough to dye his hair the same color also feels in character for jake, so you could argue either way i think
wolf keum dyes his hair pastel purple, and he's been doing it since at least middle school. this doesn't necessarily feel out of character, nor does it feel particularly in character either. wolf just does whatever he wants, period
sam “grape” lee also has purple-ish hair, but it's dark enough that you could argue he's actually naturally a raven, and the purple is just the shading (it's common to use blue or purple to shade black hair). though his nickname being “grape” means that you could argue his hair is supposed to be noticeably purple
though, at the end of the day, you can believe whatever you want about these characters' natural hair colors. it's highly likely that razen's art/coloring style for depicting eyebrows doesn't necessarily represent the characters' in-universe hair colors. plus, with the variety of unnatural/uncommon eye colors, the ‘realism’ argument is pretty flimsy.
but! fun thought experiment regardless.
