you got 2 darker grey shades on the wings that are very similar. you could merge them if you really needed to add one more shade.

only reason I really wanted more contrast was because the rear part has a shiny tone to it unless I'm reading it wrong.
but I do recon that working with these small sizes is hard.
 

Thanks! I updated with some corrections including a bit more contrast around the bottom. I am trying not to stray too far from the 5 grays I have, so I couldn't do too much without creating more clusters or adding a darker color. Maybe I should split the shade count to 3 grays & 3 slightly olive colors? That's really daring territory for me!

I've not seen Wall-E, but I am sure I'll love it and it's on my to watch list!

I like it. It reminds me of something....I think it's the Axiom from Wall-E

the steel part could use some more contrast imho, especially towards the lower part of the main body