The City uses a combination of sand-salt on roads to improve traction and melt snow and ice. The ratio of each differs depending on the conditions, and leads to variations in colour.
Salt used in the winter traction sand mix is pink because of small amounts of potassium chloride and traces of iron content in the sodium chloride salt. When combined with the sand (which also has iron), we then get a brown colour. Also, during low temperatures, we pre-wet our sand mix with liquid calcium chloride, which makes the mix look even darker.
Our sand is actually a very clean sand (less than 4% fines); the darker colour might make it look dirtier, but its appearance should not take away from the effectiveness of our winter sand.