Rust on piston head? Respected engineers Please help!?

Recently I spent a lot of money in rebuilding my 150cc sports bike's petrol engine in the showroom service centre getting and using all original components. The bike is running all fine but wasn't getting much power. The engine vibrated above 4000RPM. But Recently, while I opened the spark plug to check if the mixture is running too lean, I peeped through the spark plug hole with a white LED torch and found the head of the piston is coloured yellow (petrol colour) with many reddish brown scars on the piston head. Are they rust or something else? Am I cheated? Am I given an old piston? They took almost 9000 rupees in rebuilding the engine with a new piston, cylinder, head gasket and crankshaft. Also kindly tell me which way (clockwise or anti-clockwise) shall I turn the fuel screw of the Constant Volume Carburetor (CVC) to make the fuel air mixture more rich with fuel? Are there any problems for the engine if I run the bike with a rich mixture of fuel? Please advise.