I am posting here some settings, so that you understand the logic of the creation of custom brushes in Photoshop 7.0. For the painting of the textures, I use 5 basic brushes customized by me. The one that I am posting, is probably the most important of them... I hope that through this "mini-tutorial" I can help people put some "dirt" in their textures.
Usually, when I want to add spots of fat or rust, I use the black color as background and the brown or orange as foreground, as in the figure above:
In the sequence below, you can see the steps and settings for constructing a customized brush: