\documentclass{article}
\ExplSyntaxOn
\color_model_new:nnn { BarTone RGB } { Separation }
{
name = BarTone~555~GN ,
alternative-model = rgb ,
alternative-values = {0.1 , 0.2 , 0.3}
}
\color_set:nnn { fooRGB } { BarTone RGB / cmyk} { 0.5/1,0,1,0 }
\color_show:n { foo RGB}
\ExplSyntaxOff