I feel the new/alt rock is much better. Perhaps compromise by giving it an external outline, but no internal outlines?

Like this?



Because I just can't help myself, the outline colour is lighter around areas of lighter shade.

I love the truck! Chunky and satisfying and a bit beaten-up. Great!

I'm curious as to what's going on with the wheels though, they seem to have been replaced with exhausts. I'm sure there's a reason, and I'm not a truck guy, so...

My only comments would be:

 - The palette is a little pastel for my liking. Could be a taste thing. I'm craving more bold colours. But on the other hand it's a desert scene so it needs to look dusty.
 - The large piles of rocks in the background have the look of stacks of pancakes - they're all thin and layered over each other. I might like to see one or two with some height as well.

The gameboy version looks good, again apart from the rocks. I think that by overusing outlines you're 'wasting' one of your very limited colours. I may have a play in a bit, I haven't played with the Gameboy original palette before.

I would say that's an improvement. To me the character looks a bit more solid and less washed out.

I would like to see some bigger changes to the faceplate and shoes as well. The highlight colour on the face is so close to the surface colour that it's hardly visible except when really zoomed in. You could try darkening the main shade to allow the highlight to shine.

What is the line down the middle of the face supposed to represent? And are the faceplate and shoes supposed to be made of metal?

From the side it looks like the helmet is curved, and the glint on the front-facing version makes me think the light is coming from above left. So here's an idea for giving that helmet a little shape and volume.

You can think about this for other body parts too. It looks like you've done a lot of "copy + flip horizontal + paste" to construct your robot. If the light source is directional, this way of assembling pixel art won't give convincing results without some extra work. Think about where light is coming from and apply shading.

Just adding a glint will only get you so far. Being bolder with areas of shade and creating asymmetry with your lighting will round out your character and add solidity.

Edit: only the helmet has been edited. The earpiece on the right should probably be in shade as well, for example.

Here I've drawn some guides based on the curve of the tower described by the roof above. As you can see, the bottoms of the windows should be curved to match. At the moment it looks like they're cut out of a flat surface, which doesn't look right.

The very tops of the arches of the windows should be touching the top line. This should help work out how the arches need to distort to wrap around that cylinder.



You have the lightsource coming from a bit to the left, but the darkest shade on your tower is both on the left and the right. In fact, you're going even darker on the left.

The second example I've added here gives you an alternative that makes more sense visually. It also gives more scope for using shade in the image later to differentiate different parts from each other.



In your image it looks like the door is standing proud of the building, which is visually confusing. See my top example, as seen from above. If you push it back to make it inset, the tower will describe a curve cutting through the lintel, as in the lower example I added. This would help the image make more sense.



As the lightsource is coming from the left, A should be darker than B.



Here, the flowers at B are in shadow, but also the same shade as those at A. The flowers at B should be darker.

Hopefully this is helpful. I'd love to see how you get on... If you're interested in improving this image further, let me know and I'll point out a few other areas to work on. If you want examples of the changes themselves, just ask.