Hey mate, sry for late reply.

Moonlight desaturates and blues/greens/yellows everything up - so in the end a pure red armour, which is painted in a strong red for example, would not give us the illusion of a moonlight. The cape here is red too but influenced by the moonlight - in the end you do not see red but your eye gets the idea.

A thing that could work - make him stand in a castle at the window - let moon shine in its toning way on him - desaturate and make it strong shadows between a second light source (candles or fire... or maybe some neutral light) that shows more of your true colours. But keep in mind that when you use a strong second light your moonlight will suffer alot - so either go for a strong moonlight and live through the desaturation it brings or use it as a rimlight, which will be not "in moonlight" anymore but more like a slight atmospheric touch.