when you import your mesh with it's own uv in the Houdini , uv attributes type is Vertex by default , So you should convert it to point's attribute type before VOP SOP.
But In the expression method like tex ,If your imported mesh has it's own uv ,then Houdini use this uv by default and you don't need to convert it. (Because we use $MAPU , $MAPV for read uv )
Check my attached project (JKPointColor.Zip) in the my last post.
I made a grid in the Maya and import it with obj format in the Houdini ,and I used attribPromote SOP to convert vertex attr to point attr