I have seen some really bad results from girls trying to go too light with a home color kit. One of my friends came out strawberry blonde, which was really a better outcome than some I've seen.
Being in the pool a lot and not rinsing the hair free of chlorine can do very nasty things when you color at home.
You could always tell who had not been successful with hair color coming to school on Monday morning. Girls would have their hair tied up into a scarf or something. It was pretty easy to figure out. I've seen some gnarly shades of orange on people with formerly dark hair who should have gone to a pro. Light to dark works much better than dark to light when you're dealing with permanent hair color.