Fixing gray roots in Photoshop is easy and only requires a few short and simple steps.
1. After opening the image you want to edit, do any color adjustments that you need to do before going on to fix gray hair roots.
2. Use the Dodge Tool to softly lighten the gray roots. You will probably need to adjust the exposure to no higher than 25%.
3. Use the Healing Brush Tool in Color Mode to select an area of the hair that you want the grey roots to look like. I find that this works best when selecting a darker shade of that color in the hair.
4. Click all over the gray roots with the Healing Brush Tool in Color Mode to color the gray roots.