Prepare an ice-water bath by filling a large bowl with ice and water. Remove alternating stripes of peel from the cucumbers. Thinly slice and set aside.
Bring a large pot of salted water to a rolling boil over high heat. Place the 8 cups of vegetables in the boiling water and let cook until vibrant in color but still firm, 1–2 minutes. Drain well in a colander, and then set the colander in the ice-water bath (to set the color and stop the cooking), making sure the vegetables are submerged. Drain well. Set aside.
Place half the red onion, garlic, mustard seeds, coriander seeds and peppercorns in the bottom of a large, sealable, non-reactive bowl or jar. Transfer the blanched vegetables to the jar, layering to alternate the color and texture. Layer in remaining onion, cucumber and peppers.
Combine the vinegar, sugar, and salt in a large non-reactive 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 of the 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.