Mix cabbage, onion, green bell pepper, red bell pepper, carrots, and olives together in a large bowl.

Whisk vinegar, oil, sugar, Dijon mustard, salt, celery seed, and mustard seed together in a saucepan. Bring to a boil; cook and stir until sugar is dissolved, about 1 minute. Pour hot dressing over cabbage mixture and toss gently. Cover the bowl with plastic wrap and refrigerate for at least 8 hours or overnight.