Step 4: Fill The Hue/Saturation Layer's Mask With Black

One of the great things about adjustment layers in Photoshop is that each one automatically comes with its own layer mask, and we're going to use it to fix the problem we currently have with our entire image being colorized when all we really want is for the hair to be colorized.

To start with, let's completely hide the effects of the Hue/Saturation adjustment layer by filling its layer mask with black. Since black is our current Background color and the layer mask is already selected (Photoshop automatically selected the layer mask for us when we added the adjustment layer and set our Foreground and Background colors to white and black, respectively), all we need to do is use the keyboard shortcut Ctrl+Backspace (Win) / Command+Delete (Mac) to fill the layer mask with black.

As soon as we do, the colorizing effect disappears from our image and if we look in the Layers palette, we can see that the adjustment layer's thumbnail, which gives us a preview of what the layer mask looks like, is now filled with black:

Step 5: Select The Brush Tool

To bring back the colorizing effect and have it applied only to the hair, all we need to do is paint with white on our layer mask over the hair. Anywhere we paint with white on the layer mask will reveal the effects of the Hue/Saturation adjustment layer, and anywhere we leave black will keep the effects hidden. First, we need the Brush Tool, so either select it from the Tools palette or press the letter B on your keyboard to select it with the shortcut:

We need to paint with white on the layer mask to reveal the colorizing effect on the hair, and Photoshop has already set our Foreground color to white for us, as we can see in the Foreground and Background color swatches near the bottom of the Tools palette (the swatch on the left is the Foreground color and the swatch on the right is the Background color):

We also already have our layer mask selected, and we can tell that because the layer mask thumbnail in the Layers palette has a white highlight border around it, as shown in the image for Step 4 above, which tells us that the mask is selected.

Step 6: Paint With White Over The Hair

With our Brush Tool selected, the layer mask for the Hue/Saturation adjustment layer selected, and white as our Foreground color, all we need to do now is paint over the hair. To quickly change your brush size as you paint, use the left and right bracket keys on your keyboard. The left bracket key makes the brush smaller and the right one makes it larger. You'll want to use a soft-edged brush, so hold down your Shift key and press the left bracket key a few times to soften its edges (holding Shift and pressing the right bracket key a few times makes the brush edges harder). Then simply paint over the hair to bring back the colorizing effect:

For trickier areas where there's only loose strands of hair, go up to the Options Bar at the top of the screen and lower the Opacity of the brush down to about 25% or so:

Then lower the size of your brush so it's no bigger than the area you're colorizing and paint over it a couple of times. This way, the color won't be so intense in those areas. You may find it helps to zoom in on those areas as well, and the easiest way to do that is to hold down the Ctrl (Win) / Command (Mac) key and the Spacebar and drag a selection around the area. Photoshop will then zoom into the area you selected:

Press Ctrl+Alt+0 (Win) / Command+Option+0 (Mac) when you're done to zoom back out to 100%.

If you make a mistake as you're painting and accidentally paint over an area you didn't mean to (happens all the time), simply press X to swap your Foreground and Background colors, making black your Foreground color, and paint over the mistake to undo it. Then press X again to set your Foreground color back to white and continue painting until you have all the hair colorized:

We'll look at how to fine-tune the effect next!