Ok, it seems that I solved the issue.
I believe it was related to some operations I did last time I performed a DB backup within PSu.
If memory serves me well the steps I did were:
1) I backupped my PSU DB to a directory on my primary NAS
2) Then that directory was moved to my secondary NAS (as a part of an automatic nightly NAS Backup)
3) Then i restored the directory of my secondary NAS into a newly created DB on the primary NAS
I guess somehow PSu was tricked by the process...
To fix it the steps I did were:
1) In PSu I set the second top level folder (which has not an IP address in the name ) as the new top level folder
2) I mapped that folder on the second top level folder of the secondary NAS (the 2 NASes have identical folder structures): Psu displayed "Access violation (0)" error message but completed the mapping
3) I re-mapped the folder on the second top level folder of the primary NAS: again Psu displayed "Access violation (0)" message but completed the mapping.
Tah-dah: the folder tree suddenly showed all the missing colours of my tests (which means that the colour changes were actually recorded somewhere by PSu, even if they were not shown) and colour changes are now kept across tree compression expansion cycle.
