Hell yeah, Hex On Steroids is my go-to tool for hex editing.
The only thing i'm using for graphics is Dolphin's texture replacement feature. None of the graphics are being changed in the ISO.
Yeah, I too am certain the skin color isn't with the other player data. If you copy the complete player data for one guy and paste it over another's, the player will still have the same skin color (and general size) as they did originally. I believe there's a "player model" section somewhere in the code, but I can't find it.