From miles of walking experience getting mud on boots means that the underlying leather is wet and darker than normal.
Mud from rivers caking your boots comes in several categories:
Silt: Light almost slimey stuff seen a lot in movies around "Gold panning sites", tending to be paler and close to a dark sand colour.
Gloop: Consistancy of spilled PVA Glue mixed with sand. Mid Brown.
Farmyard: Dark Brown Smelly Cr*p, squirts out from under your foot and no matter how you put your walking gaiters on still gets inside your socks and up your trouser legs.
Clarts: (Brilliant Geordie word) Thick semi dry Clay which clings to your boots and acts as the best anchor know to creation. Guarenteed to pull off a loose boot at the first attempt. Varies in colour from Gingery Red-Brown, down Cornwall way, to Cheviot Hills Black Brown. Dries like concrete and is an absolute bastard to get off a dogs fur.