leather cuirass

[MeinPanzer]

I highly doubt it. We don't see any sort of depiction of reflection on metal in painting at this time beyond very simple stuff in complex paintings (like the shield reflecting the fallen man in the Alexander mosaic), and bronze is consistently depicted in yellow or brown, while iron is shown in blue or grey.

It's not about depicting actual reflections, as if the metal were highly polished and an image can be seen. It's about artistic observation, especially if the colour of a material, if very dull and not highly polished, appears green due to grass and trees surrounding the subject. As you say, iron is usually depicted blue; is iron actually blue?

I understand what you are saying, but comparison with other Graeco-Roman painting shows that bronze was not depicted in green, and so the most likely explanation is that this is simply the fading of the colours. There seems to have been little innovation in the depiction of metals in painting throughout this time period, and in general it seems that certain common colours were used to signal metals (yellow: bronze; blue: iron) without any consideration for environmental influence.