I ALWAYS change pistons when changing rings. Honestly, Pistons are cheap in the big picture, so replace them. Also replace the piston pin, the small end bearing and the circlips. I would never recommend re-using any of those items.

As for the rings, are they all there? Any missing chunks?

I end up with coloring on the piston skirt below the rings. The coloring I wouldn't worry about. I would focus on what looks like nicks in the piston crown/above rings. That means there was some foriegn debree up there bouncing around. THat is unless I am not seeing the pics properly and there are no nicks?
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Charles Gallant