To emphasise the contrast between air-water and fire-earth, I have chosen cold and warm colour palettes for the skin tones. Beyond the most straight and obvious references to the elements, I wanted the White Clouds to have an spiritual, ascetic appearance; evoking the spectral. On the contrary, the Black Volcanoes should look earthly, fiery, carnal. Two opposite, incompatible life styles.
The irony here is that the rust on each of their armour relates to the enemy's complementary hues. The undesirable corrosion works as an ongoing reminder of the other's existence.