metapost: gray to rgb conversion

<Layout xmlns="urn:speedata.de:2009/publisher/en"
    xmlns:sd="urn:speedata:2009/publisher/functions/en">
    <DefineColor name="mygray" model="gray" g="23" />
    <DefineGraphic name="mp">
        drawoptions(withcolor colors.mygray);
        fill box;
    </DefineGraphic>

    <Record element="data">
        <PlaceObject>
            <Box height="5" width="5" graphic="mp" />
        </PlaceObject>
    </Record>
</Layout>

this color gets converted to RGB, this should be gray.

speedata / publisher

metapost: gray to rgb conversion #524