Rinse the millet under cold water and drain. Bring 7 cups of water to a boil in a 4-quart pot over high heat. Add 3/4 tsp. salt. Add the millet, reduce the heat to a simmer, and cook uncovered, stirring occasionally and adding more boiling water as necessary to keep the millet covered, until tender, about 15 to 20 minutes. Drain and rinse the millet with cold water to stop the cooking.
Transfer the millet to a foil-lined rimmed baking sheet, drizzle with 1 Tbs. of the oil, and toss lightly to coat. Spread the millet on the baking sheet and cool completely at room temperature or in the refrigerator.
Put the vinegar and grapefruit juice in a small bowl and gradually whisk in the remaining 1/2 cup of oil. Whisk in the honey. Taste and season with salt, pepper, and additional vinegar, juice, or oil as needed.
Put the cooked and cooled millet in a large serving bowl and toss to break up any clumps. Add the avocado, orange segments, grapefruit segment, red onion, mint, and 1/2 cup vinaigrette and toss. Taste and season as needed with more vinaigrette, salt, pepper, and serve.
The salad can be refrigerated for up to 1 day. If making ahead, let sit at room temperature so it’s not refrigerator-cold and season with more vinaigrette, salt, and pepper before serving.