In an extra large 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.
Drain using large colander, rinse with cold water and drain.
In large pot, combine vinegar, sugar, 1/4 cup water, celery seeds, turmeric, and mustard seeds. Bring to a boil, stirring until sugar dissolves. Reduce heat, simmer uncovered for 3 minutes. Add drained vegetable mix and if desired, green food coloring. Return to boiling, reduce heat and simmer uncovered for 10 minutes, stirring occasionally.
Ladle hot relish into hot 8 oz sterlized jars, leaving 1/2 headspace. Wipe jars and add lids.
Process jars for 10 minutes, start timing once water is boiling.