How to remove "Yellow" tag from NAS partition?

Hi,

the other day I somehow accidentally added the "Yellow" tag to a partition that my Mac mounts via AFP from a Synology Diskstation NAS.

Even given my total experience of dealing with computers, I can't seem to be able to get rid of it again!

- In the context menu of the partition where the color labels are also shown yellow is selected, but I can't click on any of the colors to change it.

- In the "Get Info" dialog of the Finder it's the same situation.

- I have also ejected all partitions from the NAS, logged on to it via SSH, removed the .DS_Store file, and mounted again, to no avail.

The "Yellow" is still there.

Any other ideas? Where does the operating system store this information? And please, the more technical the description the better (I've got 10+ years of Unix shell experience).

Thanks