In biblical times people didn't have the ability to have pure white clothing the way we do today with our bleaches and washing capabilities.  So God used snow as an indicator of how pure he was going to wash and cleanse them. But God tell us He will cleanse and wash us to that we are even whitier than snow. God alone is the One who will wash us by cleansing us of our red scralet sins, as difficult as blood stains are to remove, but He will wash us and remove the stains of our sins. The key is submiting to Him and staying humble.