engine part staining = heat issue(provided oil is good quality, and in good shape, and more importantly, adequate in flow to the affected area)
heat is the perpetrator, oil is the victim
as scotty has said, he has seen plenty of pistons with yellowish stains(if the stains are more gold in colour, it merely means the temp involved was a little higher) and so have I and many others
I have seen it inside the piston, on the skirt, around the ring lands
it really is common(especially in air cooled engines) but it is neither desireable, nor is it to be regarded as a "normal" condition
engines in good mechanical state, with correct tuning, (esp not too lean, and timing not too advanced) should show no signs of discolouration

i remember the top of my rods being blue , that was at 1800ks , ray said it was some heat treatment HD done on the new lighter rods ????????