Nice atmospheric first piece. Welcome! =)

There aren't any major pixeling flaws in this piece, just a little banding as saby noticed (see this tuto if you haven't).
The clouds volumes are hard to read; the stars are a distraction IMO.
Choosing a bright saturated color for the sun might make an interesting visual contrast.

@ChristianLech: this up-to-date version of the checker has more features ;)
 

Looks very nice man! But actually there are 9 colors as you can see in this tool that i just found out:

http://wouterpleizier.nl/pj/index.php?input=http%3A%2F%2Fpixeljoint.com%2Fpixelart%2F106483.htm

As you can see in the results you still can reduce color numbers

Nice mileage out of 10 colours. This is beautiful, but higher contrast in the palette could compliment the picture and it would benefit from more consistent antialias use: there are places that have none and look off in 100% view (mostly the closest hill slope and the innermost glow, especially the latter due to higher contrast at that edge). There's also some banding and at the left the lighter specks at the cloud's edge look orphaned: http://puu.sh/qkFsS/717448b5e7.png Hope this helps, you have a great beginning.