by specified location or (much better) smart view rules.
ideally a smart view would include the color option, so the color would be displayed right there in the smart view tree.
i know this probably isnt practical for people with lots and lots of smart views (artists/genres/year etc) but if you use smart views to sort stuff by source, format, function or sorted/unsorted, especially if each of these smart views has several sub-views, color coding a search would be truly indispensable.