Odd Tweed Waistcoat
Wool Tartan Waistcoat
In my opinion you should have 3 basic colors and then work your way to other colors.
You can wear the basics with virtually anything. A few other colors to have are reds(bright tomato, maroon, etc), a bottle green, and maybe lighter shades of blue. Tartans should be kept minimal and make sure you know how to tone it down if it is loud. As far as tweed, whatever you like. I personally like grays and orange browns. Occasionally, I'll get a salt and pepper tweed; kidding I have 3 in my closet.
If you are layering similar colors make sure the first layer is lighter than the subsequent layer and darker than your tie. In other words, if you are wearing a charcoal suit a medium grey makes the best layer. Otherwise, just go the other way. I like to wear bright layers under dark jackets and dark layers under light jackets. The contrast is awesome!
Photo Credit: Paul Stuart, Jcrew, Everyday Elegance Blog, & a Japanese blog I can't remember.