\documentclass{article}
\usepackage{l3color,xcolor}
\begin{document}
\ExplSyntaxOn
\color_set:nnn {mycr} {cmyk/rgb}{1,0,0,0/1,0,0}
\definecolor {mycr} {cmyk/rgb}{1,0,0,0/1,0,0}
\color_set:nnn {myrc} {rgb/cmyk}{1,0,0/1,0,0,0}
\definecolor {myrc} {rgb/cmyk}{1,0,0/1,0,0,0}
\color_set:nn {mixA} {myrc!50!mycr}
\colorlet {mixA} {myrc!50!mycr}
xcolor:~{\color{mixA}mixA}\par
l3color:~{\color_select:n{mixA}mixA}\par
\color_show:n {mixA}
\tl_set:Nn \l_color_fixed_model_tl {cmyk}
\color_set:nn {mixB} {myrc!50!mycr}