- Top layer: color/50% opacity; Middle layer: overlay/25% opacity; Bottom layer: multiply/10% opacity. Feel free to experiment with different opacity settings. For example, if you're lightening dark hair, change multiply to screen.
- Insert a Hue/Saturation layer as the top layer of the group to give you more subtle control over the look of the new color if you wish.
- Group all these layers and add a mask for the entire group.