Simple recipe involving layer paints and a wash to weather muddy cloaks, capes or cloth.
Hull Red or Golden Heavy Body Acrylics/Raw Umber In this example, stipple or sponge the lower parts of the cloak (front and back) where it's most exposed to the ground. Feel free to expand it to the upper parts of the cloak.
Flat Earth Apply some using the same stippling or sponging method, this time closer towards the edge of the cloak (front and back).
Flat Earth If the cloak has a lighter side, thin with a 1:1 mix of glaze/acrylic medium or water and apply downwards in layers, building up towards the bottom.
Green Brown Lastly, a little edge highlight via stippling or sponging on the bottom edges (front and back) where needed.