Time to clear up some misconceptions.
We may not get any more snow now that April is here, but remember the big drifts of this past winter? The snow started out white and then after a few days, snow plowed to the side of the road turned gray and was dotted with pieces of dirt.
The typical explanation for the gray color is either "dirt" or "pollution" or dirt resulting from pollution. However, if pollution were falling so fast that it could discolor snow in two or three days, we would notice it everywhere -- on cars, sides of houses, clothes, shoes, statues, and so on.
Besides, the air pollution around here is tiny particulates (too small to see), and ozone smog. Acid rain would not color snow, would it?
The drifts' gray appearance is optical. True, there are motes of dirt and assorted garbage in the snow, but the gray is the result of melting. Take a look at the ice cubes in your freezer. They are not snow white, they're gray. Snow that melts and refreezes is the same.
Snowflakes, incidentally, are clear, but very reflective. Catch a flake in your hand. It's not white.
Ponder the color differences between rain clouds and fair weather clouds. The water-filled rain clouds are gray and the cumulus clouds are composed of zillions of clear droplets and appear white.
Yale research has also found that some bird feathers and insect wings are colored not by pigments, but by refracted light. Same thing with snow.
Now you know.