\documentclass{article}
\begin{document}
some text
\ExplSyntaxOn
\color_model_new:nnn{BarTone}{Separation}
{
name = BarTone~555~GN ,
alternative-model = cmyk ,
alternative-values = { 0.1 , 0.2 , 0.3 , 0.4 }
}
\color_select:nn{BarTone}{0.5} BarTone
\ExplSyntaxOff
\end{document}