in pixel art, especially at that resolution, less is more. the fewer shades you have, the more crisp the sprite will be, and the darks/lights would stand out more
take this sprite from a nitrome game for example:
notice how it uses very few shades of blue for the balloon, which makes the difference in value more significant than if it were just a gradient.
also remember to never, EVER shade in black. add some saturation to the shadows so they look less yucky
