Step 1

First off, let's start by creating a new document 600 by 600 or whatever size you want your background to be in. Now applying clouds (Filter > Render > Clouds).

Note Please make sure your background and forground is set to the default color black and white.

 

Step 2

In this step we will apply Mezzotint (Filter > Pixelate > Mezzotint) with type "Long Strokes.

Step 3

On this step (Filter > Blur > Motion Blur), with an angle of 90 degrees and a distance of 999 for the settings, everything will be better..

Step 4

Now add a new layer and overlay some color over it. Here I chose the Rainbow Gradient using the Gradient Tool (G), but you can choose whatever suits you. When you have a layer of color, set the layer mode to Overlay.

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Step 5

Ok let make this raindow brigher to make the image either brighter or gloomier depending on the mood.