I usually use neutral colors for both piecing and quilting, so I rarely have this problem these days. When it comes up I try to use them for basting, but extra bobbins are on my back-up list of things to buy when I just "have to" use a coupon for something.
Sometimes over the years I have bought old sewing accessories at garage sales and such, and it's funny how often I have found that old bobbins will have several colors of thread wound on top of each other. There would be no way to get at any color but the last one wound, so why did they do that? I guess it just felt frugal somehow. They weren't throwing good thread away, but they were making it impossible to put a full load of new thread on the bobbin. It would have driven me nuts. These were metal bobbins, so there is no way to see what colors might be on there if you're looking for one. Exposing the layers of thread is like an archeological dig through someone's sewing history.