Place bell peppers, tomatoes, onions, and cabbage in the bowl of a food processor; pulse until finely minced. Drain; transfer mixture to a large bowl. Sprinkle with salt, cover, and let stand 8 hours to overnight. Transfer mixture to a colander; rinse and drain. Return mixture to the bowl.

Inspect ten 16-ounce jars for cracks and rings for rust, discarding any defective ones. Immerse in simmering water until relish is ready. Wash new, unused lids and rings in warm soapy water.

Meanwhile, combine sugar, white vinegar, water, mustard seeds, turmeric, and celery seeds in a separate bowl; pour over vegetable mixture and stir to combine.

Transfer mixture to a large pot; bring to a boil over high heat. Boil 5 to 10 minutes. Transfer to sterilized jars; cool then cover. Store in the refrigerator.