Grunge brushes can be a fun way to create a pattern or background.
I used a very simple brush (see the inset on the graphic) and used various brush presets to create scattering and color changes as I clicked my wacom pen.
I saved this combination as a brush preset so I can come back to it when I want.
I won't go into all the settings in this post - just an example to inspire! If you want to play with these features, I'm sure there are lots of YouTube videos and tuts on brush scattering and other brush preset features.
Actually, I first created the brush scatter below and then I used the Photoshop offset filter to create a pattern tile, which I used to create the pattern above. That's definitely a topic for another post!
Here are my scattering settings.
And the Color Dynamics