I roughed this without a reference, so I'm guessing the major problem I'm having here is determining a lightsource, and actually figuring planes and depth of the formations instead of just filling in points with lighter tone to create forms that don't actually function in the space.  The orange arrows are where the lights are going to be coming from (clean, flourescent for now). 

 

I believe that's just growing pains of the work though.  What I'm more concerned about is how to transition from the lighted area of the floor to the area of shadows (yellow arrow).  I'd like to avoid dithering as long as possible, and I'd be okay adding 1-4 more colors to the palette (trying to keep it lower than not though) to make the light transitions more gradual.  I'm imagining working with some variety of non-blues could help, but stepping outside of monochromatic palettes so far has been a dicey business for me.  I'm guessing maybe breaking the ground into more uniquely heightened and lowered areas would help with casting shadows?  Breaking patches of the lighter tone into the dark area close to the rim of the shadow? 

 

Thank you for your comments and criticism.