In photoshop you can make a simple coloured image and then add texture but adding noise from the effects menu. This is a simple way to get an effect that you can control as you adjust the amount while you preview it. To tone things more generally you can use a the burn tool set up with a large soft edged brush. Set the burn tool to about 10 or 20 percent so it effects things slowly and you can control it easily.
If you don't have photoshop, it may well be that you can do this in sumopaint, online, but I have not tried it.
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http://www.cgtextures.com/
If you don't have photoshop, it may well be that you can do this in sumopaint, online, but I have not tried it.
hope that helps.
kipper