Jorge,
Here's a suggestion for some basic chipping that simulates wear through the paint down to the base metal:
I use two colors for about 80% of all my chipping - Citadel (Games Workshop) "Tin Bitz" and "Boltgun Metal." The first color is a medium metalic rusty brown and the second is a medium metalic gray.
I lay down chips of the "Tin Bitz" around hatch edges and other high wear spots. I follow up on top of the "Tin Bitz" with the "Boltgun Metal" allowing a small bit of the "Tin Bitz" to show around the edges. (Call this a "bulls eye" painting technique or method.)
This last, leaving a thin darker line around the edges, is important to give your work some depth and "3-diminsionality," and is the basic technique for most chipping.
The next step up (in difficulty) from this basic two-color chipping is to add a third color to represent the worn edges of the vehicle's paint. This is also done in reverse by adding the lighter vehicle color chips, followed by the rust color, followed by the bare metal color, allowing thin lines of each layer to show around the edges.
If you can't find the Citadel Colors, you can use any other acrylic paint that you're comfortable brush painting. Thin paint, maybe with a bit of flow enhancer and a drying retarder with good brushes is the key.
Here's a photo of a bit of 3-color chipping that I did:
The lighter vehicle color is Vallejo "Khaki" the rust color is Citadel "Tin Bitz" and the bare metal is Citadel "Boltgun Metal."
To add variety, leave some of your rust chips without the bare metal or only use the lighter vehicle color to represent scratches through the top layers of dirt and paint, but not down to the underlying metal.
German armor benefits from the occasional scratch through the top layer of paint to the underlying red primer paint. This is done the same way as the earlier two color chips but this time only with a lighter vehicle color followed by the red primer (which cold be followed by a rust color and then by bare metal for a 4-color chip if you're ambitious).
Keeping 2-color chips to a scale size is easier than with 3 or more colors, so that's why the basic chip is a simple 2-color one. After you've mastered that, then the sky's the limit.
The most "advanced" technique is to apply the lighter chip edges around the darker center chips. This just requires more brush control and ability with the paint, but represents about as far as most painters carry the effect. But using a basic "bulls eye" method (2 or 3-color) will get you about 98% of the appearance achievable with the more advanced brush control.
And this is all there is to it.
HTH,
Mike