\startuseMPgraphic{alpha}
	fill unitsquare
		xyscaled (\overlaywidth, \overlayheight)
		withcolor (uniformdeviate 1, uniformdeviate 1, uniformdeviate 1);
\stopuseMPgraphic

\defineoverlay
	[alpha]
	[\useMPgraphic{alpha}]

\startreusableMPgraphic{beta}
	fill unitsquare
		xyscaled (\overlaywidth, \overlayheight)
		withcolor (uniformdeviate 1, uniformdeviate 1, uniformdeviate 1);
\stopreusableMPgraphic

\defineoverlay
	[beta]
	[\reuseMPgraphic{beta}]

\startuniqueMPgraphic{gamma}
	fill unitsquare
		xyscaled (\overlaywidth, \overlayheight)
		withcolor (uniformdeviate 1, uniformdeviate 1, uniformdeviate 1);
\stopuniqueMPgraphic

\defineoverlay
	[gamma]
	[\uniqueMPgraphic{gamma}]

\starttext

\dontleavehmode
\framed[background=alpha]{foo}
\framed[background=alpha]{foo}
\framed[background=alpha]{foobar}

\dontleavehmode
\framed[background=beta]{foo}
\framed[background=beta]{foo}
\framed[background=beta]{foobar}

\dontleavehmode
\framed[background=gamma]{foo}
\framed[background=gamma]{foo}
\framed[background=gamma]{foobar}

\stoptext