Prepare an ice-water bath by filling a large bowl with ice and water. Remove alternating stripes of peel from the cucumbers. Set aside.
Bring a large pot of salted water to a rolling boil over high heat. Fill a large bowl with ice and water and set aside. Place the 8 cups of vegetables in the boiling water and let cook until vibrant in color but still firm, 1-2 minutes. Drain the vegetables well in a colander, and then set the colander with the vegetables in the ice-water bath (to set the color and stop the cooking), making sure the vegetables are submerged. Drain well. Set aside.
Place ½ the red onion, garlic, mustard seeds, coriander seeds, and peppercorns in the bottom of a large sealable bowl or jar. Transfer the blanched vegetables to the jar, layering to alternate the color and texture. Layer in remaining ½ onion, cucumber, and peppers.
Combine vinegar, sugar, and salt in a large saucepan over medium-high heat. Cook until the mixture comes to just under a boil. Pour mixture directly over vegetables and spices. Depending on the size container and the size of the vegetables you may not use all of the vinegar. Allow the mixture to cool to room temperature. Cover or seal and store refrigerated, stirring occasionally, for at least 48 hours. Serve well-chilled.