I seem to have the opposite experience. If I use a natural colour it never covers and I always end up with the orange/brassy still showing through. I had a dark red brown in my hair and tried to cover it with a dark natural brown.. I still ended up with red that was fading to a nice orange tint.
That's why I went the bleach route. LOL I don't know if it's just my hair or what?, but ash shades have always worked better for me.
BUT I did call my hairdresser this morning before I went out and bought a colour. She said a light brown would be my best bet to cover and hopfully match my natural color the best. I asked her about a light ash brown, she said it would be fine for the orange-yellow parts but any yellow-gold hair may turn a bit muddy depending...
So I put on a hat (LOL) and went to the pharmacy.. I compared Natural light brown and Natural light ash brown. The natural light brown result looked quite orange-ish and I immediately didn't want it.. I have enough orange. LOL Sooooo I got the light ash brown and I'm going to do another strand test. If it looks gross I'll go back and get the other one.
Sigh.. this is more of a pain in the butt than I thought it would be. Although if the bleach had of lightened my hair more than it did the medium blonde I originally bought would have been fine.