Ok, I'm going to clear up and misconceptions here.
First off box color is extremely damaging in and of itself so repeated use is very harmful.
I color my new growth every 4 weeks or so with a professional color and its pretty healthy but then again I work in a salon so it better be!
The other thing is that box color is considered a progressive dye, and what that is a dye that does not rinse out of your hair strand ever and it keeps darkening your hair.
As far as just coating the hair that is also a myth, what the color has to do is open the cuticle.
The cuticle is like a door in your house.
The developer in your color opens the door so the color can go inside.
But unlike professional products that can close the cuticle (door) the progressive dyes leave that door wide open so your hair is permanently damaged until it gets cut off.
yes there are cosmetic ways to make it look healthier but it will continue to be dull in appearance.
I think that covers most of it.