I use up my bobbins regardless of color in piecing. I DO try to match up with fabric if there is a bobbin with thread similar. I must confess that I have wound another color on top of one partially filled. Some of these I use up in hand sewing binding down or mending. I have also taken thread off a bobbin to put new thread on it if I don't have one that is a presentable match. I guess it depends on the mood I am in. LOL