Apply a heatmap to a scene

I’m using the ProgressiveLightMap class, and I would like to apply a gradient (a heatmap) to the scene to see which colour areas are more and less illuminated.

I don’t want to use any post-processing, and to be honest I don’t know where to start.

I’ve thought about taking the textures generated by the ProgressiveLightMap class, transforming them somehow with a heatmap before applying them to the objects in the scene. I’ve also seen that the LUT class may be useful.

I’m very lost with this issues, so I’m begging for some advice here.

This is how the scene looks:

You must use shaders