Here's a quick 'How To' on how I do Pigment Layering for those interested from Per-Erik Kristiansen.

In this SBS I've used colours I wanted on my model, so they are not a de facto standard to get good results.
It's important to start with the lightest colour and work your way towards the darkest one. Patience is the key here in means of drying time, but a hair dryer(not the wifes), will speed up the drying process.

First I mixed a light mix of MIG P026 Concrete, P028 Europe Dust og P037 Gulf War Sand. Put it on with an old brush and used Pigment Fixer to set it.

Step 1:

Then I mixed a 2nd darker blend. Applied this too with the old brush and used the fixer to set it as well. I t's important to re-wet the entire section so that no tide marks are left from the Fixer as it dried. The colours I used are in the pic.

Step 2:

Then I used MIG Dark Wash to re-define earlier made oil stains, stains by the wheel hubs, grease nipples etc. Not really that visible in the pic, but it's there.

Step 3:

Finally I mixed a 3rd even darker blend to put a layer down low and on the road wheels. Re-wet the entire area with the Fixer, and here's the result. The colours I used are MIG P023, P033 and P034.

Step 4:

The next step is to match the tracks with the weathering on the lower hull. Use the same approach with layering of the pigments and the same colours to make it match. Use the same layering approach on the front and rear of the vehicle as well.

Step 5:

In the picture below I've mixed pigments and Fixer, used a large brush and flicked the mix onto the model.
On top of the front fenders I sprinkled a dry mix of pigments in the same colour blend, dipped a large brush
in pigment fixer and just touched the edge of the fenders, and the capillary effect did the rest of the job.
In this step it's important not to touch the pigments with the wet brush, let the fixer flow freely and allow it to "surround" the sprinkled on pigments.

This is to avoid the "flattened" look and get some structure in the mud.

Step 6:

The same flicking / sprinkling method is used on the rear of the vehicle.