If you access, there is mention of dark grass/pale grass below the robin in the CPMN vol. 1, #3 Sep 1993 pg. 29.
I have not come across this after that.
Archer
Just saw the newsletter and found a short article. It is interesting that the note talks about two different things:
1. The small and large B on BBP and BBX. This one has been mentioned in the GPMC.
2. Differences in "colour" in notes from ARF to ARN and maybe ARP and ARS and also ARX. The prefix range that is mentioned is the same as the prefix range that I have mentioned. However the condition mentioned is the "colour" difference. I think a closer look will show that the more definitive difference is the design of the grass around the BPN and also the design of grass in some other areas.
Based on that article it is likely that ARP, ARS and ARX would have notes with both conditions.
If anyone has these prefixes ARF to ARZ (but more importantly ARM, ARN, ARP, ARS and ARX) I would appreciate if you can post a picture of the back that can be zoomed to adequately show the detail around the BPN.