Many of the hair styles do not offer black hair color, limiting our choices in character decorations. It is ironic because some hairs (actually most) of the top choicr hairs offer white instead of black color. Would it be possible to add real black hair color in hair color choices?
I'd blame Nexon on this, since they are the one who decided that "black" hairs should be "pastel white/grey" hairs just because that's what current Maplestory players wanted (along with bunch of bowlcut style for male hairs I suppose). As for the question if it's possible or not, I'd say yes, but that would require bits of works to make them look good (you could do it by putting negative contrast and all, but it wouldn't look good in my opinion).
I know current maple has mix-dye system where you can mix hair colors for customization. is it possible to adopt that system in our current client?
I'm not sure about that, but I doubt it can be implemented to the client. And even if we managed to bring dye mixing to the game, it won't be able to make a white hair into black hair, unfortunately, but it will bring a new layer of character customization, though!
