Well it's not really my place to say but firstly, most stuff is black and white and in the best scenario it would have to be categories something along the lines of "dark_hair" "light_hair". Having blonde,red,blue,green etc would only work for partly/fully colored doujin. Checking the covers (which are generally colored most of the time) might help in discerning the hair color but not all the time.
As for the button to fill in missing tags:
Sukaith has said quite a number of times that making changes to the website is a lot harder than it seems since there isn't anyone who can code the website to that extent. And also letting any random person edit tags can cause the doujin to be tagged incorrectly due to the person's inexperience or them just trying to be trolls.
As for the missing tags:
That's pretty much what the "Report Error" forum page is for. Any missing/wrong tags can be posted there and will be checked by the staff.