I feel that the skin is too dark and the two shades are too similar to be distinguishable.
How can you say that without him telling us which of the two he's supposed to be?
Three quick tips
:
The left leg is too similar in size (and shade) to the right one for it to give perspective of a running motion.
The head is too big in relation to the rest of the body (a problem, this being a realistic portrait).
The arms are kind of small, and the left one (artist's perspective) is smaller than the right one even thought the right one is supposed to be farther away).