Marnie, speak! Good girl.

Clean up sketch

Choose IMAGE | ADJUST | LEVELS

Choose AUTO. This is going to get you some pure whites and blacks on your image.

You may want to enlarge this next image to see how the Levels have been adjusted. You will need to move the white arrow quite far to the right and the gray arrow very close to the black one. Nudging the black arrow right a little will darken the lines. This may not be necessary if you used black instead of blue ink.

Since your image is in color, it'll probably still have a slight hue to it, in places, the next step will fix that up.

Go to IMAGE | MODE | GRAYSCALE to strip out the color.

Go back to your Levels CMND+L (CTRL+L on PC) and really darken the lines by dragging the black pointer to the right.

Switch your image back to RGB mode. Your image should now have a dark outline and a white background. If there are a few areas that are wonky, just use the eraser tool to clean them up.

NOTE: If you don't use a thick marker to create your original lines, it's going to be harder to get a good clean outline and a crisp white background.

I usually adjust the canvas size (the size of the whole image) at this point, being sure that white is set as my background color.

You'll want to make the page big enough that you can include swatches and notes on the side, to better explain what you are going for.

Use the brush tool (click the letter B) to close any gaps in your line drawing. If you use the eyedropper tool (click the letter i) to sample the line nearby, you'll be able to match the color. Click the closed and open brackets ([]) to increase and decrease the size of the brush to match your original drawing lines.

Notice that I also added in a missing line to the front pleat. This is your chance to get all your little details fixed up.

Quick tip: To draw a straight line, such as the line on the inside of the pleat, click at the first point, hold down the shift button, then click on the destination point. In this case, I clicked from the point on the hem to the top of the pleat.

Copy and paste the drawing to a new layer (or you can drag the background layer to the icon for a new layer in the layers palette). Set the layer blending mode to MULTIPLY by clicking the word NORMAL at the top of the palette and choosing MULTIPLY from the list. Double click the name of the layer to give it a more meaningful name.

Setting the layer to MULTIPLY makes it possible to place color below the drawing, without having to clear out all the white areas of the drawing, yet maintains the dark lines.

Block out major color areas

Add a new layer. I'm calling it, "Primary Color" as it will be the dominant color in the garment. This layer must appear BELOW the linework layer. With your Magic Wand tool (click the letter W) click an area that should be a primary color. Holding down SHIFT, you can click on any other areas that should be the same color. You may need to adjust your wand settings if it isn't selecting the area fully. If you click and area and it selects more than the area you inteneded, you may have a gap in your sketch. Use your brush tool to close any gaps.

Set your foreground color to any color at all. It's ok if it's not the actual color you want it to be, it just has to be something other than white or black. I've chosen purple.

With your skirt still selected AND with the Primary Colors layer active, fill the selected area by typing, OPTION+DEL.

If you prefer, you can fill using the EDIT | FILL option from the menu or the Bucket tool.

Repeat with the secondary color creating a new layer for it so that the two colors have their own layers.

Do this once more for the skin.

Shading

First, add a new layer over the top of the skin layer and call it "Skin Shading" or something meaningful to you. Set the blending of the layer to MULTIPLY, just like you did with the original sketch layer.

With the Brush Tool [B] , set the brush settings to about 10-20% opacity and a good soft edged brush that is about the same width as the legs in the drawing. Your options might look a little like this:

In the skin layer, hold down CMND (CTRL on PC) and select the little thumbnail of the layer. This should select just the non-transparent areas of the layer. You could achieve the same effect with the magic wand.

Go back to the shading layer and begin to add just a bit of shading to the edge of the legs. Because you selected the legs shape, the brush will not draw outside of the selection. This will allow you to gently apply the shading to just the edges. You can adjust the brush size with the brackets ([]) and can delete the shading with the eraser, as needed.

Repeat this exact same process with the other two layers, making sure that each shading layer is it's own layer above it's corresponding color layer.

If you were extra spiffy, you could certainly add a highlight layer too. With a highlight layer, you'd want to set the layer blending to SCREEN instead of MULTIPLY.

Refining your colors

So now you have a base with which to play with your colors. Let's select the Primary Color layer which is currently purple. At the bottom of the Layers palette is a little italic fx. Click that and choose, COLOR OVERLAY.

The system always defaults to red. Click the red swatch to choose a new color.

This will bring up the color picker from which you may choose a color you like. I'm going to pick a gray shade.

You can repeat this process with the secondary color. Here, I'm choosing a lilac shade.

At any point, you can go back and change those colors again, until you find a combo you are happy with.

After that, you can apply a gradient to the background, to give the illustration a little depth.

Clearly, this is a very simple explanation. As you add texture and lace, your drawing and its details will become more complicated to colorize. In all those cases, though, it's simply a matter of laying down the color on individual layers. After that, all the shading and color adjustments remain the same. You can also scan in or photograph your swatch and use that to fill in your drawing.

Once again, you can download the original sketch here and the final document here.

I hope you enjoyed the tutorial.