I'm looking at the colour on the non-highlighted underneath sections of her hair, and I don't think that's a red base. Given that she has blonde highlights, it most likely isn't - If it was, the blonde would take on some of the red and look pink when washed (which is why they aren't usually combined). It looks to me like a warm, medium toned chestnut, which may shine reddish under bright lights.
I am not a hairdresser but hey I think I do at least as good of a job as a professional does most of the time. I have to agree it does seem like a chestnut tone. I colored a friends hair a chestnut a few weeks back from a dark blond and it had that kind of reddish hue when it is in the sun. It is very pretty.
She's mentioned a few times that she uses Superior Preference's Intense Red Copper (Not that I'm a stalker either... ) I've used it too, and I would recommend it to copy Christina's colour.
Now I am flattered that I have to hair admirers.....C-dot you hit it on the head I do use that color and have to quite a few years I love it.