I have one guy client that no matter what color I use, it does not cover his grey hair!  It is very frustrating.  The best thing that I have found is to put perm solution on it for about 5 minutes, rinse it off, dry his hair and then put the color on.  I'll then put him under the dryer for 10 minutes and then start the color processing time over and have him process for the whole time.  I'm curious about the whole salt idea-I've never heard of that!