Hi,Saw an experiment done in Greenland (I think it was). They took a sheet of highly reflective white plastic, somewhere around the size of half a football field and placed it over a snow field. A few days later they returned, not sure how many days, they removed the cover and found the snow under the reflective sheet sat a good number of inches above the surrounding snow. Clearly the cover prevented sunlight from melting the snow. The sheet was pure white whereas the snow was dusty and dirty, and therefore absorbed more energy from the sun. Saw that on PBS I believe.
Oh yes the dirty snow melts faster because part of it is darker. Even a sandy color will help over pure white.