Why are riding breeches beige or tan?

If you look at English riders, you'll see that most of the time we wear beige or tan breeches. Sometimes you'll see white ones at a show, and sometimes a rainbow of colors, but beige or tan appear to be traditional. Why is this?

The answer is, as far as I know just that, tradition. White breeches, however, are generally only worn when working in show arenas. Beige and earthy colors are for following hounds or the trail.

So I honestly suspect - although I don't know - that the origin of wearing those colors was simple: They don't show the dirt and dust inevitably picked up on a long ride and still look smart at the end of the day.