I mean, we kind of do though? We could assume that the surviving images of statues showing how they were painted are accurate. If you know the colour of the underlayer, this actually lets you determine exactly what the colouration of the paint on top of that is despite it not being present whatsoever

This gives you a general trend of how brightly underlayed statues tended to be painted afterwards to finish them, and lets you infer how other statues without surviving coloured pictures of them would have appeared based on the likely prevailing style at the time