Smart shader for human body and Environment separately, also to display clothes you can use the same shader, just lower the "Power" parameter, or you can use Env shader and paint vertex colors for heat zones

Both of them can be used for any type of surface they are interchangeable

• in the Body shader all colors auto generated
• in the Environment shader brightness of vertex color is warmth (SHIFT+K in vertex paint mode to fill object color)
  
  Environment shader based on Ambient occlusion (so you need to turn it ON if you use EEVEE and tune the density and power by your taste)
  
  Update :
  Added Flir gradient colors
  Added gradient in Compositor for fake thermal imager photo effect (in TrueFlirColorsLatest.blend file)
  Added slow heating shader based on object position

The shader output is clear color so you can use it anywhere in your project :)

I highly recommend to change the ViewTransform to "Standard" mode for true colors preset