i am naturally dark blonde and recently used that stuff to bleach my hair out before re-doing my red, i had about an inch of roots and those suckers were WHITE. i was kind of shocked cause i only left it on for about 20mins?
are you dyeing from your natural colour or is it coloured at the minute?
as for the porosity (is that even a word?!) thing, i'm not sure, but if you dont like it and want to go dark, as far as i know you have to do a filler, rather than just throw some brown in