Theme color is your choice. No best color. Some users change frequently, some never. I have been using a custom dark red for a few months and really like it.
Only the built-in app you mentioned and OEM apps, like HTC, are permitted to use double wide tiles, so you won't find any. For customizable tiles, look for apps with Live Tiles. Things like; The Weather Channel, Sports Scores and Fox News will change all the time. Another is Smart Tile, which allows you to change the color and background icon.
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