Have you contacted the artist and asked them? Most are willing to share their tips and help people who ask and want to learn something new. It could just be the lighting they applied, in addition to a background for the one on the left.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi