Bad hair happens all the time, but catastrophic hair usually comes out of a box. Coloring your hair at home can be convenient and easy on the wallet, but the aftermath is just the opposite.
I understand that getting your hair colored at a salon can be expensive, especially when you have to go in every 4-6 weeks to touch up your roots, but there are reasons that people go to salons to have their color done. First of all, its absolutely necessary if you are looking for a change. If you are brunette looking to go blonde or blonde looking to go dark, its imperitive that you see a professional. Sure its easy for a blonde to put box color on and have dark hair, it's just that you want to prevent it from going too dark (even if the box you picked out is a medium brown, it doesn't mean it will turn out that way). Also if your ends are dry, in need of a haircut, you swim a lot, or heavily style your hair this can change the composition of your hair making it more porous which could affect the outcome of your haircolor as well, which is why you really should see a pro. There are so many factors in coloring ones hair that it really isn't as simple as picking up a box and getting what you want.
But if you must...
Try to stick to the same brand that you have used before. There are so many chemicals jam packed into these boxes that allergies are more common than not. Also, stay within one shade of your hair color, do not turn to box color for a drastic change. If you have dark roots and blonde hair and you put a box color on your hair, the risk of having your roots being a different color as your ends is high. Once you have your color, buy two boxes. Its better to have too much than not enough, especially if your hair is thick or long, half colored hair is just not pretty. If you hair is short (too short to put in respectable pony tail) than one box will be fine.
If you do have a mishap, please do not go and buy more color and put it on your hair. There is a reason your color job didn't work out in the first place, for a reason that is unknown to you, and putting layers of color on your hair runs the risk of extreme damage. Believe me, it's worth seeing the trained professionals in the first place so they don't have to spend countless hours and a lot of your money fixing your mistake. They have years of experience and they do this for a living, you do not. If you are afraid that it won't turn out, it's not worth the risk.
So if you're not 14 and trying to do something funky and wild to your hair, hair color should really be left to the pros.