I don't match colors in the top and bottom bobbin when I am piecing, most of my quilting is multi color, so unless I am top quilting, my Janome 10000 doesn't care if the weight or color of the thread matches. I just tried aurifil for the top thread, and I really like it, first time I could say that about cotton thread. The wonderful quilters who came before us, used what they could find, what was on hand and affordable. If your machine doesn't care, then use up your thread stash for piecing, but if you have to buy new thread I highly recommend aurifil #50 for piecing. the price is hefty, but wow, inch for inch it isn't much more than Coats & Clark or gutterman.