Ah, yes. That's sort of how I do it. I first arrange the two parts as layers and line up the shapes/lines, and then try to fiddle with the color. I usually don't use duplicate layers though. I still have never managed to make things match so well that I couldn't spot the merge in the hires version. Like for example in this picture (rather large file) you can see a vertical seam in the middle. It looked decent enough once I reduced it to screen display size, but not in high quality.