OK, if you wind a new color on top of another color, how do you get to that first color if you do want to use it? The other extreme is having a bobbin for every color of thread and some means of keeping them together. I like to have just a few bobbins and keep them clear. Sometimes I wind leftover thread back onto the spool if it is a color that I only use once in a while-I can use it to hand sew or wind it back on the bobbin if the task is bigger. I also use it for piecing when the color doesn't matter, which is most of the time.