Place the carrots in a large pot and fill with enough water to cover. Bring to a boil and cook until carrots are tender. Do not over cook, the carrots should be firm but easy to bite into. Drain and rinse under cold water to stop the cooking.
In the container of a food processor or blender, combine the tomato soup, sugar, oil, vinegar, cayenne pepper, salt and mustard powder. Blend for about 3 minutes or until creamy.
In a large bowl, mix together the carrots, green, red and yellow bell peppers and onion. Toss with the dressing, cover and refrigerate overnight. The longer this salad is allowed to sit, the better it is.