I've tried using a Karma Mtlx material builder to get the point color attribute Cd to render on the Solaris Karma side.
The obj geometry has point Cd attribute and is promoted to vertex.
Using Mtlxsurface unlit.
Tried MtlX Geometry Color vop access the displayColor primvar.
Also tried the Mtlx Geometry property value displayColor.

No luck when I render into mplay there is a solid alpha but the C is black.

Found the solution!
The assign material node needed the primitives pulldown field to be set to "all components".
When choosing that you will see this in the field

`lopinputprims('.', 0)` %kind:component