This is closely tied to the idea post from @Flopbot here, because there'd be little use for the idea in this post until the other is created.
It would be useful to me to allow a watchlist to be created on flag color. If flags were re-name-able, they would be incredibily useful dynamic tools for tracking details of transactions that change frequently. This, in turn, would make it very useful to be able to create watchlists based on those flags.