In the first is Milton fresh from his sojourn in Eternity, prepared to undertake the mission that Blake has assigned him. His lack of clothing is symbolic of his being outside of the world of materiality which he will soon rejoin.
Second we look at the image of Los, which is from the beginning of Jerusalem as our first image is from the beginning of Milton. Erdman points out that the positioning of the bodies of these two represents a mirror image. Unlike Milton, Los is clothed. This is the reversal of what we would expect if we were thinking naturally, since Milton is a human and Los is an Eternal. But the character Los, in the poem Jerusalem, is playing the opposite role to that played by Milton. He is leaving the material world to enter the stage where the Eternal drama will unfold. ________________________________
Our third image is of Urizen, clothed in a robe which seems to grow from his body as he is entering the dark world of his own creation. In other words he doesn't just wear the garment, he is the garment.
From this I understand that the degree of clothing can be used to hide or reveal the character's spiritual activity or condition.