Hey there!
There's not too much to give advice on here, as it's clear that you're just beginning. Some things that stand out to me are that in your iso piece, the little rim on the roof is thinner on the sides of the building closer to the viewer. This doesn't match the isometric perspective, and it should be the same width all the way around the roof.
For your asteroid, take away a ton of colors. You'll find that working with limited colors lets you experiment in fantastic new ways.

I wish you luck with your pixeling!