I read the instructions for Just For Men Control GX. It is a grey reducing shampoo. It has a notice that you must test a drop of the product on your skin and wait a day to see if you have a reaction. OK, I think that is on most hair dye products. Then it goes on to say if you have not used the product in 2 weeks you have to perform the test again. Why would that be?
People develop allergies to things. If you’ve been using it, you’d know if you started reacting to it. If you haven’t, they want you to test again before you put it all over your head.
I think it’s pretty common for people to have reactions to hair dye, so it’s a pretty cautious instruction.