A quick tutorial on how to change the colour of hair using the Gradient tool in Photoshop – colours can be set using the Gradient Editor
Open your image and make a new layer above the Background, calling it Gradient. Select the Gradient tool from the Tools bar and set it to Spectrum in the Gradient Editor, which is located on the Options bar.
Place a gradient from the top to the bottom of the picture. We now want to change the blending mode of the gradient layer to Overlay by selecting this in the Layers palette. This will colour the entire picture with the colours of the gradient.
We now want to add a Hide All layer mask to the gradient layer. Do this by going to the Layers menu and down to Layer Mask>Hide All. Next select a soft edge brush with the Foreground colour set to white and paint some of the colour back in.