Bob,
I don't know how to color using the PCO with a sanded in finish. I get the feeling that it may resist penetration of stains even at the very first few coats. Possibly adding it to the finish during the sand-in process? My recollection was that the alkenet stain can be created by soaking the root in mineral spirits. If so, the colored mineral spirits could be used to thin the PCO during the sand-in process.