look at your own hand, it has more than one color, right?. try to use more colors to give the hand more depth, it looks flat now. (notice you did that with the stem and leaves, they look better because they have more than one green, but sometimes they have yellows, reds, or browns too) look at your own hand again, and you'll see knuckles, tendons (and other anatomic features I don't know their names)- try giving the impression of them as well..
drawn in 19 min