Can anybody please tell me how to keep color rating and star rating in sync between C1 V 12 and PM 6.
The simple approach is to always do your metadata work in PM and have C1 reload metadata if/when you want them in C1. I don't feel that as a loss, PM is (by far IMO) the better metadata tool.
PM will pick up raw file metadata that C1 writes to sidecar XMP files, but not for DNG raw files (more below).
Color tags must have the same textual name in both applications. The PM classic (try it) green tag has textual label "Typical". That goes not spell "Green" which is what C1 recognizes. The actual color shown is irrelevant if I remember this correctly. Luckily, PM allows you to customize color labels. C1 does not. The color ordering (number 1-8) may also play a role in metadata exchange, but I can not remember if C1 is among the affected applications. My color label setup in PM (Preferences, General) for use with C1 is:
1: Red
2: Green
3: Blue
4: Pink
5: Purple
6: Orange
7: Yellow
8: Do not use with C1
In C1 (Preferences, Image) I have
Auto Sync sidecar XMP : None
Unchecked: Prefer Embedded XMP over embedded IPTC
Unchecked: Prefer Sidecar XMP over embedded metadata
Whenever I want to reload metadata (from PM) in C1, I select Reload Metadata in C1 (three dots in Metadata tool)
Whenever I want to export metadata from C1 (to PM, say) , I select Sync Metadata in C1. It works for raw files except DNGs. PM does not pick up metadata from DNG sidecars. PM expects DNG metadata to be embedded in the DNG file. C1 never updates (writes to) an image file regardless of file type, C1 Sync Metadata writes metadata to the sidecar XMP file only. I see this issue as a flaw in PM. CB knows about it, but I do not expect a fix.
You can turn on auto reload or full sync in C1 preference Auto Sync sidecar XMP. It works, but previous versions of C1 constantly spent a noticeable amount resources staying in sync so I turned it off. I never tried automatic reload or full sync or with C1 version 12.
My advice would be to start with the simple approach, then experiment with settings in PM and C1 to close in on where you want to be.