Was it true that Celts used wee to dye their hair yellow?

No. Not true. Urine will not dye hair. They did use urine for many uses. When making leather they would soak the pelts in urine, let it sit, and after a few weeks the fur could be scraped off. Urine was also used with other things as a bonding agent for dying clothing. Urine and mussel shells cooked together makes a nice color purple. Urine was also used in making gunpowder.