The easiest way to change hair color by photoshop.
1. Turn on Mask Mode (Q)
2. Use Brush Tool to Select Hair Section
3. Quit Mask Mode (Q)
4. Select Inverse (CTRL + SHIFT + I )
5. Use Rectangular Tool - Right Click - Select and Mask
- Output Settings: New Layer with Layer Mask
6. Ctrl + Click Mask Layer
7. Image - Adjustment - Hue ( CTRL + U )
8. Adjust Hair Color