To fill in a color easily, use the Magic Wand tool (W), go to the outline layer, and make a selection of a part of the image you want to fill with color by clicking on the white space that you want to color in. The black lines of the outline will prevent the selection from spilling over to other parts of the image. Then, with the selection in place, go to the layer that you want the color to be on, use the Paint Bucket tool, and fill the selection with color.

For parts of the image—such as the hair or bunched up fabric—where the outline is not simply a border but also helping to emphasize the texture of the particular part of image—such as the direction and flow of hair or the way fabric creases—then you may run into difficulty making selections with the Magic Wand tool, which will often want to grab the entire outline in the selection of the black lines of the outline are drawn too close to each other. In such cases, we’ll have no choice but to either color everything in by hand with a hard brush. Or we can use the Pen Tool (P). With the Pen Tool, we draw a path around the hard-to-fill-in parts of the image and then right click->Fill Path…

It will take quite a while, but once you finally filling the image in with color, it will look something like this.

Before we go any further, let’s pick a light source—that is, the direction the light is coming from—so we know where to draw the highlights and where to draw the shadows. For this particular image, the light is coming from the top left corner, so there will be more light on the left side of her body and less on the right.

While we could technically start anywhere with shading in the image, I usually like to start with the subjects themselves. After all, they are the life of the image—the clothes and hair and everything else are simply accessories. So let’s start with coloring in eyes, lips, and skin. We’ll make the guy’s eyes blue, the girl’s eyes purple, and her lips pink.

Now we’ll do some shading using a combination of the Brush tool (B) and the Dodge tool (O). I usually like to use the Brush tool to manually select lighter or darker shades of the same color for highlighting or adding in shadow, and then use the Dodge tool set to Highlights to create a bit of a glow around the highlights.