Man, my roots always turn brown when I do resv. changes or sometimes when I top it off. Once I had to take the plants out and left them hanging for 30 mins and they turned brown.
After reading all over the place I am starting to think that brown tinge on roots in a common occurance and should not be over corrected by nutrients, peroxcide, bleach, etc and if the root remains healthy other then the brown color and your plants look healthy dont worry to much about it.
It could be -
1.Your roots are getting some light
2.Your water temp is to high
3.Not enough exygen
Probably much more but those are the most common things I have noticed associated with the problem. I put half a cap of bleach in my water per gallon to fight algae and other baddies and still have slightly brown roots but my green is healthy green so I dont worry anymore.
The leafs will tell you the story, I think most failures in this hobby is people who mess with way to much!