this and most of your other sprites would look a lot better if you picked a light source and shaded according to it. what you have now is pretty close to what people call "pillow shading", meaning that you have the lightest colour in the middle of the object. which is actually not how pillows look either. it might help you to use something like a lamp to move around and see hot it lights different things from different angles. i''m not saying you have to be "correct" all the time, but it will probably make your stuff look a lot better if the shading is closer to reality.