over highlighted hair, want to cover highlights

I'm sorry you're having hair problems. You don't have to wait to colour again, though. Your hair won't get any healthier by waiting. Hair is already dead; it doesn't repair itself. Deep conditioning will make your hair look and feel better, but it doesn't actually repair it.

A really gentle option at this point is to use a demi-permanent like Natural Instincts or ColorSpa. I use NI medium ash blonde to tone my highlights. It gives them a nice beige cast. Depending on what colour you're aiming for, you could use a dark blonde or light brown. Just make sure you do a test strand first, though.

My initial reaction is to recommend an ash shade, but you mentioned getting a greyish tint on your lightest highlights. Most ash shades are violet based and I'm afraid those strands might pick up a purple tint. It would fade, but you'd probably be very unhappy with it for the first week or two. Maybe you should try a test strand with a neutral light brown.

Another option is to do a test strand with a permanent colour. A while ago I put Nice 'N Easy Light Ash Brown over a bad bleach job. I didn't notice any new damage, but it made my hair a dark walnut colour which faded to a golden brown within about 3 weeks.

Whatever you do, make sure you take the time for test strands first. It's really worth the extra effort. I ended up with apricot hair at one point because I was too lazy to do a test strand.

Sorry. I'm rambling. I hope something I said here was helpful to you. Good luck.

What about visiting a hair salon? A good colorist could put a nice semi over your highlights and the cost probably wouldn't be too expensive. You'd also be more likely to get the results you want.

I don't know how people color their own hair. There is just too much that can go wrong when you do it yourself. Heck, there's a lot that can go wrong even when you go to a salon!