Sparkly – I don’t know. I don’t think there’s a halachic issue. I think it’s the type of thing that some people might disapprove of. Personally, I think that if you don’t have grey hair and you are stam dying your hair from brunette to blond, it might not be a good idea, because it could give people a wrong impression of you.
On the other hand, if you have grey hair and have to dye your hair anyhow, it’s kind of like “why not just dye it the color you want?” but still, it could look funny to others. I would suggest going slowly – like maybe, the first time you dye your hair, make it a bit lighter than usual, and each time, make it a bit lighter, but I wouldn’t go from total brunette to total blonde.
However, that is just my opinion. I don’t think there are halachic problems, and if you think that in your circles, people won’t talk about you or have the wrong impression of you, do what you want.