This technique (which I learned from Kevin Ames) provides the most control and flexibility for modifying or adjusting hair color, because it uses a layer mask and adjusts the layer so it does not "damage pixels," which is a "non-destructive modifier."
First step:
Open the photo you want to decorate and select a color balance from the Create new Adjustment layer pop-up menu (half black and half white circle icon) at the bottom of the layer palette.
Step Two:
After the dialog box pops up, adjust the sliders and adjust the color to the hair color you want. Select each option in the tonal balance section of the Color Balance dialog box, and then move the color slider to adjust the shadow, middle, and high light. In this case, we want to turn her hair red, so for the shadows, the middle and the high light, we're going to move the top slider to the red side (move to +16, +74, and +45, respectively). Then click OK, this will make the whole picture has a very heavy red color bias.
Step Three:
Press X until the foreground color turns black, then press Alt-backspace (Mac:option-delete), and fill the color balance mask with black. Doing so will eliminate the red hue in the photo.
Fourth Step:
Press B to select the Brush tool in the Toolbox, click the thumbnail on the right side of the brush text in the options bar, and then select a soft corner brush in the brush selector. Press D to set the foreground color to white, and then start drawing on its hair. The red color bias added to the front with color balance is redrawn when drawing. After all the hair appears color deviation, please go to the Layer palette, the color balance adjustment layer Blending mode from normal to color, then reduce the opacity, so that hair color looks natural (about 50%).
Before and after processing comparison: (click to see larger picture)
After processing before handling
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