Once a basic color is determined, now comes the fun and creative part. She may suggest adding lighter highlights around your face and maybe even dispersed throughout your hair. Lighter color near your face can change your entire appearance without the permanent commitment of an all over change.
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.