Rust color on top piston ring...but not rust??

So, being bored all done with my yz250 refurb I figure I would just out of curiosity pull the header pipe and take a peek up in there with a flash light. Piston looks good from what I can see, I am pretty sure I can see some carbon build up maybe on the top of the piston with the piston at very bottom of travel, at first when looking at the top piston ring on the right side there looked to be rust but after I cycled the piston down and up one time, seeing lube on the piston, that discoloration went away, so I wonder what this was, it was little rust color specs that diluted away after a stroke of the piston. Also looking at the cylinder the cross hash marks are very visible and look great. Thanks for any input.

Oh and not loosing coolant and using Yamalube 2r. Does this stuff attract moisture? I live about 500 yrds from the ocean, is there a good way to store the bike while not riding. I use an exhaust bung.

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