! Improper type.
\starttext
\startuseMPgraphic{myname1}{color}
pickup pencircle scaled 1mm;
draw unitsquare scaled 1cm withcolor \MPvar{color};
\stopuseMPgraphic
\startuseMPgraphic{myname2}{color=red}
pickup pencircle scaled 1mm;
draw unitsquare scaled 1cm withcolor \MPvar{color};
\stopuseMPgraphic
\useMPgraphic{myname1}{color=blue}
\useMPgraphic{myname2}{color=orange}
\stoptext