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.
In any case, it's certainly true that coloring your locks can change the way you look. It can revitalize things taking them from boring to fresh and new. Which is why so many of us have wholeheartedly embraced coloring our locks. You just want to make sure the change is for the better.