I think what we need to do here is look at spider man, so first get me some pictures of spider man!
https://i.ytimg.com/vi/Q2vFUuiCbaM/maxresdefault.jpgSo lets try to get the pose down first. I don't wanna stray to far from your original pose (although I am going to flip the legs around to make it a little more energetic), but were going to look at the picture for the proportions, the shapes of his muscles, and the overall energy.

That's looking pretty Spidery.
So we've got the pose down, the next step would be to block out the colors, while were doing this, were going to keep the light in mind.

The next step is the hardest, and to make it looks really good you gotta know a little bit of anatomy. But were gonna shade this bad boy using only the 4 colors that are already on the sprite! (+1 gray for the chest)

Quick tip - try to break the muscles down into their own shapes, the shoulder is shades almost like a seashell, the the arms aren't just sausages, they're multiple sausages. The bicep is a sausage, the tricep is it's own sausage, the forearms got a few sausages going on there too.
Now we can call it done, but for me I like to take it one step further and break up that outline a bit, and tweak the colors to have a little more hue variation.

Done!
Quick tip - when breaking up the outline, don't just replace delete it, try to mix in a little bit of the shaded color to give it a nice break.
Bonus tip - For easy color picking, on darker shade drift toward one color on the hue slider, in his case I chose purple (always a safe bet btw) For the lighter shades stay within your desired color's hue.
Hope this helped!