\definecolor[ColorName][%
r={\mathexpr[.3N]{\getvariable{ColorName}{r}/256255}}, g={\mathexpr[.3N]{\getvariable{ColorName}{g}/256255}}, b={\mathexpr[.3N]{\getvariable{ColorName}{b}/256255}},
]%
\definecolor[ColorName][%
r={\mathexpr[.3N]{\getvariable{ColorName}{r}/256255}}, g={\mathexpr[.3N]{\getvariable{ColorName}{g}/256255}}, b={\mathexpr[.3N]{\getvariable{ColorName}{b}/256255}},
]%