Add the potatoes to a large saucepan or Dutch oven. Cover the potatoes with cold water and add 2 tablespoons of salt. Place over medium heat and bring to a simmer. Cook until the potatoes are tender, 10 to 15 minutes. Drain the potatoes and set aside.
To the same saucepan, add the vinegar, sugar, mustard seed, turmeric, black pepper and 1 teaspoon salt and bring to a boil. Add the cabbage, cauliflower and bell peppers and return the mixture to a boil. Add the potatoes back to the saucepan and toss to combine. Transfer to a bowl. Season with salt and pepper. Serve warm or refrigerate until ready to serve.