In a large nonmetal bowl combine zucchini, onion, and sweet pepper. Sprinkle the pickling salt over vegetables. Pour enough water (about 4 cups) over to cover vegetables. Cover; let stand for 3 hours. Transfer to a colander; rinse and drain well.
In a 8-or-10-quart pot combine sugar, vinegar, the 1/4 cup water, the celery seeds, turmeric, and mustard seeds. Bring to boiling; reduce heat. Simmer, uncovered, for 3 minutes. Stir in drained vegetables and, if desired, green food coloring. Return to boiling; reduce heat. Simmer, uncovered, for 10 minutes.
Ladle relish into hot, sterilized half-pint canning jars, leaving 1/2-inch headspace. Remove any air bubbles by sliding a nonmetallic utensil down the side of the jar; wipe rims and adjust lids. Process jars in a boiling water canner for 10 minutes (start timing when water begins to boil.) Remove jars; cool on wire rack.