How to Make It
Pulse zucchini, yellow squash, bell pepper, and onion in a food processor until chopped and uniform in size, 3 to 4 times.
Stir together sugar, cider vinegar, rice vinegar, mustard seeds, celery seeds, and salt in a Dutch oven, and bring to a boil over medium-high. Stir in squash mixture, and bring to a boil. Reduce heat to medium, and simmer, stirring often, 5 minutes.
Pack hot vegetables and liquid into hot sterilized jars, leaving 1/2-inch headspace. Remove air bubbles. Wipe jar rims. Center lids on jars, apply bands, and adjust to fingertip-tight. Place jars in boiling-water canner. Process as directed for 15 minutes.