In a large non-reactive bowl combine zucchini, onions, and sweet peppers. Sprinkle with salt, toss gently to coat.
Add enough cold water to cover vegetables. Cover and allow to stand at room temperature for 3 hours.
Transfer vegetables mixture to a large colander set in sink. Rinse with cold water; drain.
In a 8 quart stainless steel, enamel or nonstick heavy pot combine sugar vinegar, the 1/4 cup water, celery seeds, turmeric, and mustard seeds.
Bring to boiling, stirring until sugar dissolves, reduce heat. Simmer uncovered for 3 minutes.
Add drained vegetables mixture and if desired green food coloring. Return to boiling, reduce heat. Simmer uncovered for 10 minutes stirring occasionally.
Ladle hot relish into hot, sterilized half-pint jars, leaving 1/2 inch headspace.
Wipe jar rims, adjust lids. Process in boiling water bath canner for 10 minutes (starting with the canner water returns to a full rolling boil) Remove jars from canner.
Makes 5 half pints.