I'm not really sure what sort of help you were looking for, but I've made a fist using the same reference for you to look at.
Main things I tried to achieve was a more distinct silhouette with more emphasis on the knuckles and a more natural slope to the thumb.
Then with shading, I just really focused on the highlights and shadows to try and emphasise the form of each knuckle and digit as much as possible.
Finally, I think your colour ramps were too similar to each other, so I unified it into a single ramp and hue shifted each value on the single ramp.
Hope it helps
