Arg, I pointed you to the wrong shadow tiddler. I should have suggested that you look at $:/core/ui/TagTemplate.
Apologies !
Please find attached a file for you to download, drag into TiddlyWiki.com, and study.
This is my way of setting up inheritance.
I have "Some Tiddler" tagged with "Tag Category 1".
I have the "Tag Category 1" tiddler (i.e. the tiddler that matches the tag) in which I've specified a "color" field and selected a color that will show for that tag.
I have "Some Other Tiddler" tagged with "Test Tag 1".
And I have the "Test Tag 1" tiddler (i.e. the tiddler that matches the tag) in which I've specified a "Tag_Category" field, and set the value of that field to "Tag Category 1".
The intent is this: when a tag's matching tiddler has a "Tag_Category" specified, I want that tiddler to inherit (well, use) the color specified in the tiddler specified in Tag_Category. If the tag's matching tiddler does not have a "Tag_Category" field, then I want that tiddler to have the color specified in the color field. If it has no color field, then it will have the default color for tags.
The magic happens in the edit I've made to the $:/core/ui/TagTemplate.
Look for: colour={{{ [<currentTiddler>get[Tag_Category]get[color]] [<currentTiddler>get[color]] +[first[]] }}}
The original code was: colour={{!!color}}
Cheers, and Best wishes !