Wow, nice Coloring, I really love it's soft shading. The background's pretty damn good for being scratch (although this is just an assumption that you drew the background, but I'm preetty sure you did). The grass looks really really, but the stone doesn't seem to look very um.. "stone-ish" . Maybe if you lowered the color just a bit and make it less brighter, and more darker. Last thing, I adore the way you color in the eyes, they just go so well with the drawings, nice job.
PS: I totally admire you for being able to draw under such conditions... An airplane, you've got to be seriously talented.