Create a new image, press D then X to set the parametres of foreground and background. Then apply Filter > Render > Different Clouds.

Then select Select > Color Range as shown below:

And press Delete to remove unnesessary color, then press Ctrl + D

Now, ctreate new layer and plcae it under the layer with clouds and fiil it with blue color.

Now select our last layer and make Blending Options -> Color Overlay: