Tutorial: Coloring in GIMP

October 27, 2017

I wrapped up my illustration class this week; it's been fun to see everyone's artwork and help them on their way to new artistic endeavors! To finish up the class, I did a digital coloring demo in GIMP. I decided GIMP was a good program to show them since it's free so there's no super expensive investment to start with.
Since I already had everything typed up for the class, I decided to share it on my blog, as well. Without further ado, here are the steps behind this ink-to-digital illustration!

What you’ll need:

Preparing the Linework

Scan your linework at 300-600 DPI 
A good rule of thumb is that the finished file should be 300 DPI for color images, 600 DPI for black and white linework. Even if you’re going to color your image, you get a better result if you scan the linework in at 600 DPI and make it smaller later.
If your image has some light color in it (colored pencils, non-photo blue, etc), you can remove some of that with:
Colors> Desaturate > Luminosity
Next, adjust the levels to get rid of the gray lines and make your black lines firmer:
Colors> Levels > [pull arrows on both ends of “input levels” towards the middle to remove gray; watch the preview to see when your image is mostly pure black and white]
This next step will turn your image into only black and white. You must have only one layer in your file to do this step:
Image > Mode > Indexed > Use black and white (1-bit) palette
If there are things to fix in your linework, now is a good time to fix them with the pencil and/or eraser tool. Get rid of stray dots, crooked lines, etc. You can also select areas of the drawing to move or adjust them.

Now turn your image back to RGB so you can use colors again:
Image > Mode > Colors RGB

Next, take out the white background so you can color behind your linework:
Colors > Color to alpha > From [white] to alpha

Now there will be no background (the transparent part shows up as a gray checkerboard) so add a white background in a new layer:
Layers > New layer > Layer fill type = White

 Coloring the Image

Now that you are working with color, you may want to save a 600 DPI copy of your black-and-white linework, then reduce the linework to the finished printed size at 300 DPI.

Using the G’MIC filter is an optional step, but I think it makes things a lot easier in the initial “flatting” stage, where you’re just filling your lines with flat color—especially if you are coloring with a mouse!

If your image has lots of closed-off spaces (like a coloring book) the G’MIC filter will work well. If your image has lots of broken/open lines, you will have to do a little more work.
Filters > G'MIC > Black & White > Colorize lineart [auto-fill]
Play around with the settings to see what works best with your linework. For this image, I used:
contour threshold = 100%
minimal region area = 100
tolerance to gaps = 10
Click “Apply

Now you’ll have a lovely patchwork of psychedelic colors on a layer behind your lines:
Grab the paint bucket tool [fill similar colors] and start filling those patches of color with the color you want them to be!

You will have to fix some things. There will definitely be places where the auto-fill didn’t divide an area correctly. There are a couple of ways to fix this:

Free Select (lasso tool)

Use the lasso to select the area you want to color, then use the paint bucket to fill in the selection.
Do not check Antialiasing or Feather edges, as this will make the edges of your colored areas fuzzy, and it won’t be easy to select and area and change colors later.

Pencil Tool
Use the pencil to color in the area. Make sure you’re not using the paintbrush tool, as it will give you fuzzy edges.

If you don’t want to use the G’MIC auto-fill, you can use the free select and pencil tools to color in your lineart.

Here I've started to unify the patches of color and play around a bit with a color scheme:
Once you have your colors blocked out, it’s easy to change all instances of one color to another. Use the Select By Color Tool [Do not check Antialiasing or Feather edges] and click the color you want to change. Every patch of that color will be selected, and then you can use the paint bucket tool [fill whole selection] to re-color them all at once.

Now I've decided on my colors and have everything colored in:
To put in some details (highlights, shadows, pink cheeks, etc) you can make a new layer:
Layers > New layer > Layer fill type = Transparency
then use the paintbrush or airbrush tool to put in your color.
Although I decided not to use shadows on this illustration for a simpler, flatter look, I usually add shadows on a new layer:
Layers > New layer > Layer fill type = Transparency
Change the layer mode to Multiply or Burn, and lower the opacity if you wish.
With the select, brush, pencil, or airbrush tool, paint in your shadows. Experiment with the shadow color that works best for your illustration. I often like to use a gray or light grayish purple.

You can color your linework so it’s not black. With the linework layer selected, click the little gray checkerboard [lock alpha channel] at the top of the layers menu tab. This will make the transparent areas stay transparent.
Now you can use the paint bucket tool [fill whole selection] to change all the lines, or use the Free select or Paintbrush to make some of the lines a different color.

Here I've added a bit of color to her lips and cheeks, and turned some of the linework a lighter color to give a bit of depth:
If you want to add a texture over your illustration, you can either copy the texture image and then paste it into your image as a new layer:
Edit > Paste as > New Layer
or open the texture image directly as a new layer:
File > Open as layers > [choose your file]

Once you have your texture in a new layer over your illustration, change the texture layer’s mode at the top of the layers menu tab. I use:
Mode: Overlay
Opacity: 40%-80%
(this depends a lot on the texture and the look you want—play with it!)
You may need to use the move tool to move the texture around and get the best placement.

I used a different texture on each of these illustrations. The left texture is one I made for my George and the Dragonvine illustrations (you can grab the texture here!) and the right one is an ink/tea texture I made and used for this Howl's Moving Castle wallpaper. I've been intending to put on the blog...so check back sometime for that! :)
And there you have it--the whole illustration, start to finish. :)

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