The Goughnuts Safety Guarantee states that if your dog chews through the outside wear layer of any Goughnuts ring or stick, green, black, or orange, to expose the red indication layer, then Goughnuts will replace your toy.
The guarantee also applies to all other Goughnuts products, including their Heavy Duty items, however, the rings and the sticks are the only toys that have a red safety indicator. Full details here.