@Zeus The Swap Shop is located in your Menu:
You will find the Color Change Pass in there and when you select it, a menu will come up with items that are eligible to be used to obtain it, ie. Recycle Badges (assuming they have been fixed) and items that came from the AC scratch which have a green border around their icon like this:
Once you have a Color Change Pass, keep it in your Inventory and enter the Salon; you should now be able to change your CAST's color or the color of a recolorable Costume/Outerwear. Once you go to exit the Salon and confirm the changes, then the Color Change Pass will be consumed; if you have Premium, you have 60 minutes of in-game time to make further adjustments to the colors when going back into the Salon before the change is finalized.