\setupcolors[state=start]

% all colors are in RGB color space (and should stay there)
\definecolor[rgbgray] [r=.5,g=.5,b=.5]
\definecolor[rgbblack] [r=0,g=0,b=0]
\definecolor[rgbwhite] [r=1,g=1,b=1]

\startuniqueMPgraphic{foo}{size,color}
    fill unitsquare xyscaled(\MPvar{size},\MPvar{size}) withcolor \MPvar{color};
    draw boundingbox currentpicture withcolor \MPcolor{rgbblack};
\stopuniqueMPgraphic

\starttext

\uniqueMPgraphic{foo}{size=2.5cm,color=rgbgray} % unwanted gray conversion (mkiv)
\uniqueMPgraphic{foo}{size=2.5cm,color=rgbblack} % unwanted gray conversion (mkiv)
\uniqueMPgraphic{foo}{size=2.5cm,color=rgbwhite} % unwanted gray conversion (mkiv)
\color[rgbgray] Gray text. % ok
\blank
\color[rgbblack] Black text. % unwanted gray conversion (mkii)
\blank
\framed[background=color,backgroundcolor=rgbblack,foregroundcolor=rgbwhite]
{White text} % unwanted gray conversion of background color (mkii)

\stoptext