I think we already have a colorization tutorial but I want to show you all another technique for colorization. We will explore the use of gradient maps in this tutorial. This is the way I first learned to colorize black & white images. It is a little more difficult but the effect is great.
The 1st step is to start with a black & white image that does not have high contrast, especially on the skin areas. We want to make the image look like it was originally a color photo. After choosing an image that will make a nice colorization, open it in photoshop. Be sure the mode is set to RGB color. Image/Mode/RGB on the tool bar. Just check the RGB Color option. You can not colorize a B&W image without doing this 1st. My understanding of gradient maps is this. The gray scales are a gradient of black to white. When using a gradient it will cover the blacks with the foreground color and the whites with the background color. Now we want to color the skin first so we will choose skin tones as in Image 01. This is just going to be the main skin tones. We will use another Gradient Map (GM) to make her skin look natural a little farther along in this tutorial. Before we can add a gradient map we have to set out foreground color and background color.
Image 01:
Now that we have our colors set we can add the Gradient Map.
Image 02:
When you create a GM you will get a pop up window. Check the Dither box and OK. Now We have an image with a sepia tone look.
Image 03:

Don’t worry, we will change all this. Notice the 3 areas circled in Image 03.
#1 is the layer style. We will change this after we mask out the areas we don’t want colored with this GM.
#2 is where we will do the above.
#3 is what we will do this with.
We will use the eraser tool to remove the color.
Making sure the GM mask (#2) is active, set the foreground color to white and start removing the color leaving only the skin. You can use a big soft brush here to rough it out and a small (#5) to clarify the edges. Remember to mask as close to the lines as possible.
You should have an image that looks something like this now.
Image 04:

Now that we have the skin colored we can set the layer style to “Hard Light”. You should an image that looks like this now.
Image 05:

Now we want to make the girl look real. She needs a little color in her cheeks and around her eyes. Her teeth need to look more natural too. Remember when adding more layers the order is important. If we want another layer for some pink we need to be sure it is above the skin tone layer (Gradient Map 1). We can use more gradient maps for this or create new layers with the style of color or soft light to accomplish the life giving hues to her face, teeth and hair.
Image 06:

I added and named more layers to give the image some life. Notice how the layers are named. The top gradient map (Gradient Map 2) is for the soft red hue of her dress. I add a little pink to the cheeks, nose & chin. I added a slight blue hint around her eyes. Purple would even work here. I added a layer of brown to her hair and some brown for her eyes. Here is the final product.
In closing, there are many ways to do a single thing in photoshop. This is just one way to colorize. There are more ways but I can get more realistic looking skin tones using this technique. If nothing else, this tutorial may cause you to explore some of the options in Photoshop that are seldom used. Good chopping.
Yer pal, Keb