Hair color isn't always about just "color". It's about shades, hues, and a good cut that will show off your new color. Perfect color doesn't come in a bottle. If you are hoping to get a color correction for past mistakes (caused either by you or another salon), it will take a real expert to add in the right toners to counter the faulty shades you are trying to get rid of.
Still if that sounds at all appealing you might first want to stop and ask yourself some questions. Or at least think about these points which can help you decide among the many hair color options. Since the goal is find out which will actually work out best for you and your hair.