It depends on the original colour, but choosing one that looks relatively natural is the best option (so if you're dark haired, you're unlikely to get a natural blonde look). Other than that, blonde, brown and red all work when done well.
First I would say that her natural hair colour looks best, but that is never going to convince a teenager. When I was a teen, I wanted my mum's auburn hair, but ended up as a light brunette. She was happy for me to use red hair dye but it never took very well.
I think the best colour for her to pick is one that will suit her complexion and natural hair colour. What a lot of people forget is that you might dye your hair, but your eye brows are going to remain the same colour. Dark eye brows with a light hair colour looks fake, weird, and unattractive.
If she wants to go for something unnatural like green or blue or something, mke sure it washes out quickly in case it doesn't look good. Dark complexions suit deeper colours, while fair skinned people should avoid really bright colours.
Would point out that it may be a problem for work, or at her school, and that she may need to cut it off should these things be a problem down the track. If the school didn't care and she didn't have a job on the horizon, (wasn't trying for one) then I'd say go for it - who cares? When your a kid, that's when you can do all that sort of stuff!