It took me a long time to actually start adding color to my projects and when I did, I used a new layer and selected a color and brush then painted over the underlying picture. I then fiddled with blend modes and opacity. This is okay except it is difficult to go back and change the color if you decide it does not suit, particularly when you start blending them. This is where the Color Fill layer wins hands down.
I should add a rider here that this is how I use Color Fill layers at this time. Adobe Photoshop is a huge program and there are any number of ways to do anything and it depends on the artist and the desired outcome. Experimentation will yield amazing results!
You will find this little beauty under Layer - New Fill Layer - Solid Color. You can name this layer whatever you like or leave it default. In this example I wanted to add some of the red tinge in Angel's face shadows into Spike's face shadows. You can select a color from the color picker or use the eyedropper to pick from the image, and then say okay. Shock horror, this will cover your entire work space. Don't panic. Notice that in your Layers Panel, this new layer has a layer mask. Click on this mask, change your foreground color to black, select the Paint Bucket Tool and fill your work space with it. Voila! You image is back. That's because black hides/reverses your changes on an adjustment layer. Way cool!
A much simpler method is to use Ctril I on a PC or Command I on a Mac, while the mask is active and it will automatically change. You will also find the invert command on the mask's property box.
Now, still with the layer mask selected, change your foreground color to white and, using your preferred brush, I mostly use the standard one, you can start painting in your color where you want it. White adds your adjustment changes. I will sometimes enlarge the image at this time, to work on small areas like the lips or iris. Depending on what it is I'm doing, I might reduce the opacity and flow of the brush or I might leave them at 100%. In this project, I reduced the opacity of the brush to 25% and left flow at 100% and I used the default soft brush. I left this layer at Normal blend mode and 100%.
I then added another Color fill layer to add some white to Spike's face to again match the lighting in Angel's. I used the same brush settings but this time I changed the layer blend mode to Luminosity and reduced Opacity to 85%. The Blend mode I most often use on color layers is Color and Hue. But you should experiment until you find the result you like best. I've found Luminosity mostly the one I use for a white fill.
I only used the two Color fill layers on this image but you can use as many as you like, where you like, particularly when you want to blend colors. I typically have a separate Color Fill layer for eyes, lips, teeth, different items of clothing, eye shadow, patches on backgrounds, etc and separate layers for each figure in the image. You get the picture. Making a color fill layer for each area you want to color, even if they might be the same color, allows you to go back and change each layer individually, if you find you want to later on.
The images used in this tutorial are screen caps from the television show Buffy the Vampire Slayer.