I use whatever color (as long as it doesn't show thru) I have for piecing because no ever sees it once the quilt is done. Sometimes it will be cotton, and sometimes poly that also doesn't matter to me for piecing. When it comes time to quilt, I like to use connecting threads cotton, but also have been known to use poly. I quilt with whatever color hollers at me and says "Use me, Use me!". And horror of horrors, the top and bottom threads may not match....LOL. I do use older thread after a strength test. I refuse to spend megabucks on thread....I simply cannot justify it.