Hi, i found this way that was pretty simple.
If you create a sphere, delete the bottom half and scale it up so that it covers your scene. Apply a lambert shader and a ramp texture to it. Give the ramp 4 color divisions (I think i used dark blue, light blue, white and light blue from the top).
To get a cloud type effect you can add a fractal to the ramp colours. increase either the u or v to strech the fractal horizontally. there you go. Just play around with the settings.
This is something i did a while ago with this sky effect
Hope it helps