She doesn't necessarily dye it. First of all, I think--and correct me if I'm wrong--that that picture from Leipzig is actually from two years ago, in 2000, and if you look at other pictures from around then, Wimbledon for instance, her hair looks lighter. So it would seem that the color fluctuates, which to me says:
A) It gets lighter when she spends time in the sun. Not likely, but possible--happens to me quite often.
B) She gets highlights, which is sort of like dying it but not quite. I think this is probably the explanation. It would also explain why her hair seems lighter now, because if you get highlights steadily for any prolonged period of time, your hair pretty much takes on the color of your highlights. It would also explain why there was never an exact moment when her hair went from darker tio lighter blonde, because it would be a gradual change.
I mean, she might dye it. I have no idea.
I think I just spent wayyy too much time thinking about this.