% A group with no role would silently fall back to neutral gray, violating
% the "outline in the composite's own hue" rule. The package must error.
\documentclass[border=2pt]{standalone}
\usepackage[en]{supertensor}
\stsetrole{x}{stTeal}
\stdim{T}{4}
\begin{document}\begin{tikzpicture}
  \strow{R1}{T}
    \stgroup{G}
      \stface[role=x]{A}{}{T}{T}
      \stface[role=x]{B}{}{T}{T}
    \stgroupend
  \strowend
\end{tikzpicture}\end{document}