Shred green cabbage, red cabbage, and carrots with a knife or mandolin. Rinse thoroughly and use a salad spinner to remove excess liquid. Set aside.
Super easy option: Purchase two 16 ounce bags of tri-color coleslaw mix (without dressing packet) to save time! You'll need 28 ounces for this recipe.
Wash and finely chop the green onion. Add it to the cabbage mixture.
In a large bowl, combine mayonnaise, vinegar, maple syrup, Dijon mustard, celery seed, salt, and black pepper.
Add cabbage mixture to bowl, and gently toss vegetables with tongs to coat well.
Cover the bowl with plastic wrap and place it in the refrigerator for 30 minutes to 1 hour before serving.
Store in an airtight container in the refrigerator for up to 3 days after preparation.