This "green" color also doesn't have to be taken literally. But they would have painted literal pictures of such in order to explain the metaphysical. For instance, check Malidoma Some (The Healing Wisdom of Africa) and his meeting with the Green lady after he was told to tell his elders teh true nature of the tree:
"The tree that I had been watching for so long was no longer there, and in its place was a beautiful green lady. I do not know if the tree became her, or if she stepped out of the tree, but this really doesn’t matter. Where the tree had been there was now a figure that looked like a human being, in the shape of a woman, very tall, probably seven and a half feet tall. Her tunic was silky and black and she wore a veil over her face, and when I looked again, she had lifted the veil, revealing an unearthly face. I call her the green lady because she was green, her skin was green. But the greenness in her had nothing to do with the color of her skin. She was green from the inside out, as if her body were filled with green fluid. I do not know how I knew this, but this green was the expression of immeasurable love."
So here we see that green is often times the expression of immeasurable love. Which is definitely what Osiris represents.