I “overprocessed” one with a purple product :ambivalence:.
The royal look took several weeks of shampooing every tIme I was at he barn, fours times a week at least. I finally just used Orvus. I tried all sorts of DIY type suggestions, it faded gradually but did fade out.
Recommend not using blueing or any of the blue or purple whiteners right before a show. Use something every couple of months to keep stains from setting in year round. Look on the bright side, at least you didn’t get the bluing on he horses skin and leave it. The hair falls out, That takes a long time to grow back and looks worse then a temporarily blue or purple tail.
Oh, working a detangler into the tail hair once a week or so sure helps to keep stains from setting in. Mane hair too if you have one that thinks a fresh pile of poop makes a great pillow.