Here's a little tutorial I made on how I make that grainy wash for my backgrounds.
For most of my backgrounds, I like to make things a little less boring and have it be more dynamic. You’ll find that you can add a lot of atmosphere this way. Of course, when it’s concept art, emphasis should be on the subject, and not the presentation, but what the hell. It looks cool if you got the extra time, and want to add that little bit of mood. It doesn’t take long at all anyway.
First, will be making a new brush. The longest part of the process, but the good news is, you’ll only do it once. If you download my brush set on my website, it's already included as "Grainy Wash." Take your default digital airbrush and tweak the fallowing in your brush controls pallet: Step 1;Change the subcategory under the “general” menu, to grainy soft cover. step 2; Make the brush big. Somewhere in the area of 80 pixels. also, raise the grain way up to 100% and drop the opacity to 5%. Step 3; Change the following under the expression sub menu:
Now the rest under opactity should be set to “none”
Now that the brush is done, save and rename it to something like “Grainy Airbrush.” Now comes the fun part. Pick a paper texture you like and go over the whole canvas with it. Tweak size, contrast, brightness, etc, as you see fit.
Pick another grain and blotch in in randomly. Repeat that step as many times as you like.
When this is done, select brightness and contrast in your effects menu and lower either to where you like them.
Now, create a new layer, set it to colorise, and simply, with your fill tool, paint it the color that you want.
Now, the last part, go into the effects menu, under the “surface control” sub menu, and choose “apply lighting.” Fool around with it until you like the way it looks.
Any feedback and comments (revisions?) are welcome.
If you do decide to try it, please, feel free to post the results.