\setupcolors[state=start]


\definecolor[rgbgray] [r=.5,g=.5,b=.5]

\startuniqueMPgraphic{foo}{size}
    fill unitsquare xyscaled(\MPvar{size},\MPvar{size}) withcolor \MPcolor{rgbgray};
\stopuniqueMPgraphic

\starttext

\uniqueMPgraphic{foo}{size=5cm}
\color[rgbgray] Gray text.

\stoptext