How to edit texture with color?

Let’s say you have a cube. On one side of the cube you added a Star Wars rebel icon. (It’s pure red). It’s a texture.

Now,you want to take a brush and add some grey color to the texture to make the icon look like it’s old and scratched.
How would you accomplish this?

+Bonus. How can you change your brush type? So instead of painting with a circle, how could you paint with a ‘scratchy brush’. (Like a brush that produces a scratching effect, like a cat scratching a painting with it’s paws!

Thanks

And this would have to be done in Blender exclusively? No external image editor (Photoshop/GIMP) allowed? Then you might want to search for texture painting tutorials in Blender, like e. g. this one.

Or you could use a scratch texture to modulate the visibility of the rebel sign decal, like so:


How this is done depends on the render engine you intend to use.

+Bonus. How can you change your brush type? So instead of painting with a circle, how could you paint with a ‘scratchy brush’. (Like a brush that produces a scratching effect, like a cat scratching a painting with it’s paws!
Use a brush texture http://wiki.blender.org/index.php/Doc:2.6/Manual/Textures/Painting#Texture

I love this forum! Thanks so much!