Welcome, nice stuff! The biggest thing I would start experimenting with is your palette. Your ramps are very conservative, you're really just varying brightness. Think about saturation and hue when you are deciding how to draw your shadows and highlights! even gray surfaces have blues and greens and reds in them in real life.
EDIT - I changed my mind, there are a few things you should be focusing on - your shading is I have to guess based on a tutorial or something rather than a real understanding of light, shadow and volume, and your use of outlines needs a LOT of work, they are mucking up otherwise fine images! Here is a quick edit to show some basic things you can do to help get around the simpler problems:
This isn't the be-all-end-all of course but I'd definitely start on these basic elements - palette and outlines are very important!