You're not thinking of the volume of the thing! That is why I made the gray bubble man, because it shows you how light would interact with the shapes he is composed of. I used the rudimentary shapes (spheres and cylinders) because that's all he really is, just with some variation. Try thinking of those shapes when you are putting the colors down. I am not going to make an edit because Jad has made a good one, and it demonstrates all these things well! Try making an animated gif that goes between your and Jad's edit, and you can see how he changed things. Then, figure out why, and then apply what you figure out.