The {{thing}}
is the transclusion.
But it sounds like you want to change the color of a tiddler based on some criteria. If your criteria is a tag, for instance, than the easy way to do this is with a stylesheet.
For instance, say you have tiddlers tagged "Yellow".
Then make a tiddler and tag it $:/tags/Stylesheet and put this content in it:
[data-tags*="Yellow"].tc-tiddler-frame {
background-color: yellow ;
}
Save the tiddler.
Now all your tiddlers tagged "Yellow" will have a yellow frame. This is the entire frame, and not just the body which is just the text part. The actual body is identified as .tc-tiddler-body
In general, real templates are used to add display functionality to tiddlers. They don't change the existing way the text is presented. There are many things that can be accomplished just with stylesheets and CSS.
HTH