Sorry but I do not think you are correct, even the official ITF website shows there is NO purple belt:
8.2 Belt:
1. The belt is five centimeters wide and five millimeters thick. It is worn wrapped around the body once.
2. There are six colors of belts:
Black: 9th to 1st degree
Red with black stripe: 1st grade (kup)
Red: 2nd grade (kup)
Blue with red stripe: 3rd grade (kup)
Blue: 4th grade (kup)
Green with blue stripe: 5th grade (kup)
Green: 6th grade (kup)
Yellow with green stripe: 7th grade (kup)
Yellow: 8th grade (kup)
White with yellow stripe: 9th grade (kup)
White: 10th grade (kup)
http://tkd-itf.org/pub_web/ver_eng/bl-pp.html
So even though some clubs as you say do have a "purple" belt, even though I have been to many and never seen one, it is not officially an ITF belt. Maybe in other disciplines but not in ITF as I originally said.