Make a CSP config (ext_config in skin folder, so it only affects that particular skin) that changes the shader. In assettocorsa\extension\config\cars\common there are templates for CSP shader changes. You select which material(s) are to be changed (in this case it's Carpaint, the exterior paint of F1 GTR) and the config values can be adjusted to fine tune its looks.
I use CSP configs skins all the time now, it offers a huge amount of possibilities. For example, the rims of Porknose, AE86, Type C could not be painted in bright colors because the material always makes it look dark, so I changed the rim material with a CSP config to one that allows bright colors.