In a covered 5- to 6-quart pan over high heat, bring about 3 quarts water to boiling.
Remove ends and any strings from green and yellow beans. Rinse beans and add to boiling water. Cook, uncovered, until beans are tender-crisp when pierced, 3 to 4 minutes. Drain, then immerse in ice water until cool, 3 to 4 minutes. Drain again.
In a wide salad bowl, stir together mustard, vinegar, olive oil, and chopped chives. Add beans and mix with dressing. Season to taste with salt and pepper. Garnish with whole chives.