Hello there, if you are sewing a seam that is two different colors, like blue and green, should you use both colors to sew the seam?
I think it would make the seam bulkier and more noticeable. Here's an excellent tutorial on how to seam. The one you may be interested in is how to do an invisible seam. The teacher in this video seamed a white yarn piece with a single strand of bright orange yarn for instruction purposes, but you couldn't see it when she was done.