I use quilting thread in the top and piecing thread in the bobbin. I like using variegated thread for FMQ, in colours that tone with the top. I've got a goldy/tan/cream one that I use a LOT! In the ditch or echo quilting I'd normally use a neutral - cream in a light quilt or grey or beige in a darker one. :-D Can't help with the invisible I'm afraid, never tried it.