How to edit or add tag colors
The tags shown when saving or modifying a file are Finder tags, not specific to Preview, etc. Therefore you are limited to the colours available for Finder tags.
Go to Finder → Preferences, then choose the Tags tab.
Click on a tag, then choose – to remove it.
Tag colours can be changed by right-clicking on a tag in the list and choosing a new colour for it.
Drag your favourite tags to the area below to make them available for quick use in Finder menus.