I have white ones for reining and I've tried everything the best I've found is take them to the car was and pressure washer just don't get really close but I found it gets the sand and mud out of the boots then just hang to dry!
Hose em down! use some soap and water if that dirt won't come off. hot water works a lot better at getting off grime then cold water FYI!
When I've used them I usually brush them off with a dandy after every ride, or hose them off if they'll have time to dry.