Is " Just For Men " hair colouring really just for men only ?

Ya know, I can't imagine that it is. Dye is dye, right? How does dye be specifically for men or for women? It just doesn't make sense. (to me) The only thing I can can think of, maybe one difference could be that, for men, there's less product in the box, due to the assumption men'll have shorter hair. Now, of course that wouldn't be it, though. Can't imagine what else it could be.  Pretty sure, also, that there wouldn't be the array of colours for the men, as compared to the women. Neither is a reason for the dye to be catering strictly to men.

Just like most other things, I bet the men's is cheaper, even if same amount per box.

Maybe there's something I'm not aware of, but for me, it's a NO.

It doesn't last as long as other dyes. Most men tend to have shorter hair or keep the beards trimmed, so it will be cut off before it fades too badly. So while anyone can use it, you just shouldn't expect the longevity of the ones marketed towards women.