You can paint XGen attribute values onto the vertices of a bound mesh using the Paint Vertex Color Tool. For example, painting a mask or color sets these values on a per-vertex basis directly on the mesh. Note that the resolution of the map is dependent on the resolution of the bound mesh. If you require higher resolutions, use you can paint a Ptex map, which uses texel resolution (see Control primitives using Ptex maps.
The XGen Expression Editor displays the following expression:
$a=vmap('${DESC}/colorSet1/');#vpaint $a