You will need.Acrylic Resin.
Pigments of varying type.
Plaster of paris or similar.
Mixing vessel.
Mixing stick.
Gloss varnish.
Fresh mud, 25% plaster, 25% acrylic resin, 15% gloss varnish and 35% pigment.

Mix up.

Apply to tank.

Et' Voila.
Simple isn't it. :)
Okay... There are a few things to remember:
When things get wet, they get darker. Dark, wet, fresh mud is nearly black.
Mud splatters and coats stuff, but it washes off some places and not others, think about how the tank moves. I put some mud vertically on the front edges, I see a Chimera rolling and rocking up and down hills and troughs into mud and bog.
Dry mud is very light, and will only remain in the crevices as it's not sticky. Unlike fresh mud.
Dust coats everything, all plates, but not the edges.
SC