Combine vinegar, oil, sugar, mustard and celery seed in a non-aluminum saucepan. Stir over medium heat until sugar dissolves and dressing comes to a boil. Remove from heat; season with salt and pepper. Cool completely.
In a large bowl, combine cabbage, onion, green and red pepper. Add dressing and toss to coat. Cover and refrigerate at least 2 hours. This may be prepared a day ahead of time. Makes 6 servings. Enjoy!