Embroidery thread for the bobbin is usually white(if the piece is not reversible, and is very fine (40wt) so it won't create bulk in the many stitches of a design. The colors would be usable on top if you don't mind a "no sheen" look--which poly & rayon give. Personally, I use the serger cones for piecing all the time. And like you< I have tons of them --some came from factory sewing and I don't even know what fabric items they were used for. But if it's still strong--it is still usable!