+TIP: DO YOUR HAIR IN BLACK AND WHITE+
Its easier to design hair in black and white than with colour PLUS this method only works with black and white. If your design is already coloured “Desaturate” the image by pressing Shift + Ctrl + U or Image > Adjustments > Desaturate.
Flatten all your hair layers and duplicate the image. (You now have a black hair colour, time to create the other colours)
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Then go to Image > Adjustments > Colour balance… or CTRL + B while the new hair layer is selected.
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Now slide the middle bar all the way across to green. Click on ‘Shadows’ & ‘Highlights’ and do the same. Then click OKAY!
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You now have very green hair. Don’t worry this isn’t the last step xD
Then go to Hue/Saturation. (CTRL + U or Image > Adjustments > Hue/Saturation…) Slide the bar around until you find a good shade of brown/blonde/red.
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Now you have a light coloured brown hair! (You may want to adjust with saturation and lightness to get it perfect)
To turn this light brown dark, use the curve tool (CTRL + M or Image > Adjustments > Curve) and adjust according to whichever looks pleasing. Use image as a guide!
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And there you have it! This is how I recolour my hair!