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Your right on with the food coloring, blue is usually the best. You can get the stuff in bulk at most bakery supply shops. For sprayers you want Hand pump/backpack type sprayers used in gardening, or landscaping. Buy the rebuild kits, and extra nozzles. Check out home depot, or any major hardware/gardening store. Also mix in some anti-freeze with that.....yeah its bad for the environment, but it also wastes an hour of time having to go down and defrost your pack.
Also to keep your nozzles from freezing, keep using them up the lift, and around the hill untill you get to the park. Just spray in the air. Keep your skis on, and just work on the technique to spray. Straight lines take a while to learn how to make.