There just photoshop-colored linearts. why not try placing the pixels by hand, the results will be a lot cleaner and better.
Welcome to the forum Nice linearts. In my opinion the shading is the biggest problem about these pieces. Seems very pillow-shaded. Try to use more intresting Lightsources, instead of shading it as if the viewer was the lightsource. Also you could try to find more intresting colour-ramps. Learn hue-shifting and stuff like that.
@hue-shifting:Hue shifting is a common technique to make colourramps more intresting and/or realistic.Here's an example:The ramps have colder colours in the darker tones and warmer colours in the brighter tones.