The Good Housekeeping Institute breaks down why: "Other than henna, any commercially available hair dye store_bought for home use or found in salons uses chemical actives for them to work," says Birnur Aral, Ph.D., Director of the Good Housekeeping Institute's Health, Beauty and Environmental Sciences Lab. "By and large, these chemicals are synthesized substances." Even when the packaging claims to be all_natural, organic or chemical_free which is literally impossible because everything, including organic things, are made of chemicals that could basically be an outright lie. This is because the FDA can't do anything about the use of these terms regarding cosmetics.
The last thing you want is to run out of dye halfway through the process. If your hair is shoulder length or longer, you may need two boxes to cover your whole head, depending on hair thickness. (And if you end up with a leftover box, you can always use it next time) Take the strand test : If you're trying a new shade, test it on a few trimmed hairs or hidden pieces first, and look at the result before you commit. A common horror story from testers who didn't do this: hair that turned orange and purple!