In another place I was asked how I muddied the shields.
Here's how I went about it:
I used Mig European dust.
Applied using Vallejo Matt Varnish, I dabbed this where I wanted the mud to appear.
The effect I was aiming for is troops fighting in a muddy field, or splattered with mud as horses pass by...
When the Mig dust has dried, you can scrape off the excess with a sharp scalpel.
Stuart.