How much Damage from Demi Color & Toner??

How frequently are you thinking of using the toner or demi-permanent color? 

The most gentle toner for bleached hair is the Born Blonde Series by Clairol, you can find them at Sally Beauty.  These usually wash out in one to two washings, but if you only shampoo your hair a couple times a week you could get by with using a bottle once a week. 

If you are going use a demi-permanent color that might last longer depending on how porous and damaged your hair is--although it may be more damaging--do to the peroxide and ammonia content.  Make sure you strand test using a demi prior to deciding to do the entire head. 

Another thing to consider is using a toning mousse.  Igora Conditioning/Toning mousses are nice, however, I have not ever used them on an entirely bleached head but they seem rather gentle and tend to tone my highlights nicely.

Another thing to consider if you are pale yellow or yellow is to get purple shampoo and use that once a week to help tone down the brassiness. 

Have you looked into the Robert Craig colors, these contain no ammonia and are made for toning bleached hair or using to just refresh virgin hair without lifting the color. 

You'll want to avoid peroxide/ammonia which is what causes the damage/dryness.

Good luck and hope this helps.

Rebekah