Used cam wear pattern

Flat tappet cams are not ground square. The cam profile is ground with a taper across the lobe width wise. This causes the lifter to rotate along with the lifter face being convex shaped, due to this the lifter face won’t hit square all the way around the cam lobe. You could pull the lifters one at a time and make sure there are no vertical lines on the lifter body. That would indicate they’re not rotating. Me personally I wouldn’t sweat it too much. The discoloration is varnish build up where the lifter doesn’t make contact. That’s my best explanation but there are definitely others here more knowledgeable.