That's stratospherical clouds Evil-Ville ... I am refering standard canonical clouds viewed from the ground :

• http://www.flickr.com/photos/marialuisa/240395611/
• http://www.flickr.com/photos/luckyplanet/129673950/

Well, to be true, in your picture, the clouds are massively blue because the photo is taken from a plane and clouds reflects the main light source : sky. When you're on the ground, the clouds reflection is nearly off, there is only the sun diffuse light that cross the cloud, thus the only blue you may see comes from the clouds transparency at the edges : very light blue.  

You may read this tutorial about clouds and lights which is quite usefull : http://www.itchy-animation.co.uk/tutorials/light03.htm

Anyway, in my opinion, the blue clouds are off.

If you want quick suggestions to improve your piece :

• there are way too much colors for a small piece : I count 9 greens, and you only use 3 for the hill shading ... The dithering technic could be improved, I suggest you to follow tutorials,
• The clouds are not blue in reality ... it's white, gray, orange, purple but not blue ...
• You shouldn't AA the outer border of your hill with dark green, but dark blue ...

Apart from that, it's a good start ... keep on pixelling !