https://animals.howstuffworks.com/pets/how-to-dye-dog-hair.htm
https://www.cuteness.com/article/dye-dog-food-coloring
I started going grey early and used hair dye for some years. Oddly enough the hair greying slowed down and hasn’t changed much in years.
I started getting bad reactions to the dye, and then read about how toxic hair dye is. Use of it for a few years may have precipitated some other health issues that started hmm.. around the same time frame.
Mairdie’s links look good! I hope no people use human hair dye on animals - they shouldn’t even use it themselves!