Got this information from J.Hottinger (ex Paris Racing)
Hope it serves you.
AFM

IDEAL ENGINE TEMPERATURES:
There is no IDEAL temperature for any engine. There are many variables that affect it; Ambient temp, fuel type and nitro content, altitude & barometric pressure, Pipe/manifold type and setting, clutch setting, glow-plug, gear ratio, available traction, How hard your driving, on road or off road Etc. Etc.
All of the following conditions MUST be met before any temperature can be considered correct.
The engine is assumed to be in good shape.
1} There must be a visible trail of smoke when accelerating from EVERY corner.
2} The idle is stable.
3} The glow plug wire stays somewhat shiny and the coil stays round [UN-DISTORTED]
4} The performance is good.

READING YOUR PLUGS:
Actually when a new plug wire just goes slightly gray after a 5 or 10 minute hard run it means your very close to an optimal horsepower tune, but be careful the next step is TOO LEAN!
1} Wire and surrounding bottom of plug wet, with like new shiny wire = rich side of optimum power 85%
2} Wire and surrounding bottom of plug starting to dry and wire starting to gray = Very close to optimum power 95%
3} Wire and surrounding bottom of plug dry, wire totally gray but not distorted optimum power 100%
4} Wire and surrounding bottom of plug dry, wire distorted = slightly lean DANGER!
5} Wire and surrounding bottom of plug dry, wire broken and distorted or burnt up = extremely lean possible engine damage!