discoloration

Hello All, was fixing a pinhole leak in my 2" copper drain pipe (from kitchen) and noticed discoloration along the underside of the pipe; most of pipe is dark/blackish, but there is a shinny (like new copper) 'vein' running along the length. Pictures are attached. Don't know if this is...

I live in Chandler, Az with very hot summers. My house is 6.5 years old and in the past 6 months I've had two very slow leaks cause ceiling damage. In both cases, it seems like there is hard water sediment on a hot water Pex line near a manifold. I had a few plumbers comment on the color of...