Queenbarbiej, most try to pick a thread that shows up the least once pressed. That being said, many don’t like changing thread out frequently. I generally use a light to mid grey for most piecing. It tends to blend with most fabrics, but sometimes it’ll be necessary to change to suit your fabrics. For example, I’d use a dark grey with blacks just so I can see the line I’ve sewn more easily. My eyes aren’t what they used to be and the dark grey will blend well. If I’m sewing black and white together, once again I’d pick a mid grey (most likely) for blending’s sake. If you stitch a seam and it’s very noticeable once you’ve opened and pressed it, consider a different color.