There is no direct telling about light or darker colored pistons, it depends the fuel. The higher the volume of the castor oil the darker the piston will get.

But if you have a brown color it seems to be OK, when it has a rough gray surface it is to lean or the compression is to high.
And what you can reed is the length of the exhaust, on the piston you can see a ( shaped mark, that has to be somewhere between the exhaust port and the middle of the piston.