The "grit' is actually rock salt which is mostly sodium chloride with some other minerals. Sodium chloride crystals are clear and the colour is caused by the other minerals. When the rock salt is crushed on wet or snow covered roads by the traffic, the salt disolves forming a brine solution which has a lower freezing point than pure water. When your shoes get wet it is with brine. The water evaporates leaving behind small crystals of salt which appear white.