Okay, so after a few hours tinkering with it and considering everyone's advice, I think I figured out the basics when it comes to layers. Basically, I just duplicated the original layer, and hardened the lines on the original. Seems to have worked, or so I hope.
I also experimented with some shading and blending of colors to create more realistic-looking tones and hues. The only part I'm not overly fond of is the ground. I tried to make it look somewhat like carpet, but...yeah, I don't think I succeeded there.
Anyway, here's my second go at digital painting. Advice and criticism, as always, are welcome.
*Uploaded both original and colored versions. Credit for the line art goes to the talented Arania.