Ray, I look for the right color on spark plug first (I like to see light cafe latte color), then on piston top. Piston top for me is more a matter of looking for signs the mixture is off (wet/dark = too rich; signs of detonation (pits, peppery metallic looking deposits) then looking for color to determine the optimal jetting (too hard anyway looking through spark plug hole with LED light).
As long as the piston top shows the engine is jetted within the ‘normal’ range (i.e. showing no signs of being too rich or too lean) I then make small jetting changes and decide on what feels best / most strong while keeping an eye on spark plug color and piston top to make sure I am not getting too lean.
I am by no means an expert engine tuner that finds the exact magical jetting every time but what I do at least ensures my engine runs good and safe.