I use only 2 colors for piecing - a cream for lighter fabrics, and dark grey for darker fabrics. If I were piecing light and dark, I'd use the cream. I've not had a problem with threads showing in the seams. Could it be you're ironing the seams open instead of pressing them? And are you setting the seams before pressing them open? If you iron them, you take the chance of stretching the fabric too taut. Setting the seams first, then pressing them open should avoid that problem.