Gas turbines have thermocouples placed just downstream from the last stage of turbines but before the afterburner (if it has one). Their purpose is to give an indication of the turbine blade temperatures because it's a bit hard to put a thermocouple directly on the blade . Gas turbines are run very close to their destruct parameters so a little too much heat weakens the blades beyond the point they can resist centrifugal forces. It's been so long now since I've worked on them that I can't even remember now what temps we were allowed.

With model engines the temp won't decline going lean because by doing that you've reduced the oil flow so friction heat builds up and there's less fuel for internal cooling. Infrared guns rely on emissivity (the black or white that Sport_Pilot mentioned) but apparently the highest rate of heat rejection by radiation comes from a natural aluminium with a satin or cast finish. Even anodising gives a heat barrier.

I use a contact type thermocouple that plugs into my tacho/multimeter and put the probe at the base of the plug. This is almost exactly the same distance from the centre of the combustion chamber for all engines and away from any cooling fins so it's likely to give a more comparative reading between different engines. I only take temperatures for my own interest and I'd never try tuning an engine by them.