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.
Is your hair healthy enough to withstand the chemical onslaught some coloring products produce? Or could it use some TLC before you step up to a new color?