Dyestamps are currently one-time use items, the colours of which are permanently lost if overwritten with other dyes at the outfit station. However, I feel that if the dyes in the stamps were permanently unlocked if you used them, it would DRASTICALLY increase the overall dyestamps sold to the overall playerbase, because people would view them as a long-term investment rather than a one-and-done luxury item.
Come on, ZOS: more dyestamps sold equals more money for you; more dyestamps sold means more dyes for us. It's a win-win. Do this.
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