To a large mixing bowl, add purple cabbage, green cabbage and carrots. Toss to combine and set aside.
In a medium mixing bowl, whisk together mayonnaise, buttermilk, sugar, apple cider vinegar, celery seed, ground mustard, salt, and pepper.
Add the mayo mixture to the cabbage mixture and toss to combine.
Transfer to the refrigerator to chill for 1-2 hours or until ready to serve.
Store in an airtight container in the refrigerator for 3-5 days.