Firstly, create a new image in the size of your choice.
Next, set your colours to the default ones by pressing D
Then, go to Filter -> Render -> Clouds
Next, go to Filter -> Pixelate -> Mosiac, use a cell of about 20.
After that, go to Filter -> Stylize -> Glowing Edges, use a setting of 2,14 and 8 respectively.
Now's the time to colour the image.
Add Hue/Saturation by either using CTRL + U or go to Image -> Adjustmensts -> Hue/Saturation and check the colorize box, then just fiddle with the Hue, Saturation and Lightness, the Hue will change the colour, while Saturation will change the amount of colour and Lightness will change the amount of light.
After that, you're done.