Does Just for Men really just color the gray hair?

I read somewhere that Just for Men colors just the hair that is gray and leaves the rest of it alone. Does it actually do this? I have a patch of gray hair that I want to color over but I want the rest of my hair to be the original color. Will it do this?Does Just for Men really just color the gray hair?
i heard it,s suppose to do it but be careful your not allergic to it.

Fish