This is a cosmetic bug. If you have the year-long VIP your dragon is purple. If you purchase an 30 VIP (recent holiday sale where you get an extra 400 gems!) while your year-long VIP is active it extends your VIP for 30 days. So far so good.
The bug is that the purchase also immediately changes your purple dragon to a green dragon. Not critical but when posting screen shots it does take away from cosmetic value of what you purchased. I would compare it to buying and avatar and then the avatar showing something other than what you purchased.