Depending on what sort of ground your horse is on, the dirt sticks better at different times.
Some mud (like red clay) seems to really stick and stain more than sandy type soils. And even brown/gray clay seems to stain the hooves some what.
However, I find that pens that don't have the manure removed often, seem to really dirty the hooves and make a brick like coating stick to the entire hoof, at least in my experience. They also tend to be the same owners that never pick out their feet, so the hooves are typically rancid with thrush, and those owners tend to "Stretch" the times between appointments, say instead of 6 weeks, they won't call until at least 9...totally aggrevating. Not saying your pens are foul or that you neglect your horse. Just a trend around here.
But yes, in snow, or really wet, tall grass, the dirt seems to get washed off, even their socks are whiter. It just sticks to the rough spots on the hoof wall, if they are that dark in the summer and you want to clean them, get a stiff brush and some soap..though that can dry out the feet, but it will come off. =)