Contrary to popular misconception, white hides dirt very well, so long as it is a fairly uniform coat. My white Accord still looks pretty clean even when it is covered in dust. Dust is usually closer in color to white than black, for instance. When you wash it, the difference between where you have washed and not washed is obvious, but without that contrast, you really can't tell from 15 feet away that the car is anything other than clean. You even still get pretty clear mirror reflections in the panels. The only exception is if you get black mud or tar thrown up onto the body.
However, my fiancee's gunmetal gray 350Z is in another league of dirt hiding. She commutes along muddy farm roads and yet you can't tell the car is dirty until you get right up on it. Obviously it doesn't "gleam" like it does when clean, but it does't look obviously dirty, either.
My "pearl" blue S2000 shows dirt considerably more than either of the other two.