Combine zucchini, onion, green and red peppers; sprinkle with salt; cover with cold water. Let stand 2 hours.
Drain; rinse and drain thoroughly.
Combine sugar, cider vinegar, mustard seed and celery seed in a large saucepan. Bring to a simmer (180 degrees).
Add vegetables; simmer 10 minutes.
Pack hot relish into hot jars, leaving 1⁄4 inch headspace.
Add 1/16 tsp of pickle crisp if desired.
Remove air bubbles.
Adjust two-piece caps.
Process 10 minutes* in a boiling-water canner.
*check any adjustments for your altitude*