In a nice running engine the underside of the pistons will remain a golden color. The hotter a piston gets it starts burning on the oil, this is the dark color.
Basically it means the pistons got hot, you know that already though.
The piston pins loosen with temperature. Use a penetrating oil to see if it will get into that tight pin. You can warm the piston with a propane torch and see if it does free up with moderate temperature. Maybe the oil will get in better when warm also.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.