Hi Doc Drew, I'd suspect that if a scratch has a different grit coarseness and reflects light differently, it might very well appear to etch differently than the surrounding matrix.
Only guessing, but I don't think the various etching protocols are being used as a 'staining' process. The etching may be used for it's ability to remove some materials at a higher rate than others so that the resulting contrast can be visualized. The resulting image may(?) be all or predominantly steel that some other component or process cause it assume a shape that can be viewed? I'm guessing that by using different etching protocols of known capabilities, they can create an informed opinion of what reacted to the etch, but may not actually be present in the image.
