Comics are a form of art, and colouring them is an art on its own. We can use many techniques: watercolour, gouache, pencil… Here we focus on a technique that looks easy, but turns out to be among the least intuitive, although perhaps having the most diverse possibilities.
First of all, we need a line-art drawing, and we can make it in many ways, such as pencil on paper or Illustrator. I chose pencil on paper and I digitized the drawing at 600 dpi.
There is an important thing to notice right away: the colour mode. I work keeping in mind that one day I might want to print my work, and then I choose to work with a subtractive colour model, usually known as CMYK. If I am sure I'm not going to print my work, I can safely choose the additive model, also known as RGB, which allows to choose colours in a different range, some of which are almost impossible to represent in print. For this work I chose the subtractive model which, depending on the target's colour profile, approximately produces duller fuchsias and greens. Just for the records.
Going back to the drawing, I have drawn the shapes with a red pencil and marked the details and contours with the classical graphite grey. What we want to do now is to keep the grey strokes and make the red ones white. Photoshop is a powerful tool because there is always more than one way to do the same thing. In older versions it was necessary to use a channel mixer layer, while in the most recent ones there's a converter that can simulate photographic filters: I used the latter, which is found in Image > Adjustments > Black & White. If we were good at scanning, it may be sufficient to select the red photographic filter and leave everything else as is. My piece of paper was a bit dirty and dark so I had to fine tune the filter, as well as fix the result using curves and levels: if you are a bit confident with Photoshop, you know how to do these things.
Once we've got a clean drawing, we must isolate the black contours. There was a time when I used to set the blending of the drawing layer to Multiply, and paint on the underlying layer, but there are cases where it can be useful to also paint the stroke, like in this example, and so we take a slightly different ride. In the Channels palette, making sure we have the CMYK channel selected, click on the button shown in the figure.
Doing so, we get a selection that faithfully reproduces the pencil drawing, and then we can go back to the Layers palette, create a new one—I called it ink—and select Fill from the Edit menu, making sure to use black.
Discarding the selection (CTRL+D) we will have a transparent layer with the black strokes. We can get back to the background layer, select all (CTRL+A) and delete the content, taking care of choosing a background colour that won't burn our eyes and at the same time that doesn't blend with the colours we're going to use.
At this point we begin to paint the colours on intermediate layers between the background and the pencil drawing. I tend to create a layer for each different item, for example one for your skin, one for each piece of clothing, and so on. It's also possible to collect them into groups, which is something I usually do. For example, in this case I created three groups, one for each character, plus one for the well. Eventually we'll get a nice Mucha-esque (below on the left), just less elegant :-)
This is the time to bring the drawing to life, that is the time to create the shadows and lights, give depth to the characters, painting textures, and so on. The advantage of having separate elements on multiple layers is that we can now lock their transparency and mess around at will. I used to abuse the round soft brush as well as the Color Dodge and Color Burn blending modes. Not that it's all wrong, and in some cases may also be acceptable, but certainly it means limiting oneself. For this reason I tried to learn the use of colour tones, so as to give even more depth and credibility to the objects that I paint, and above all I learned how to use brushes. Photoshop is a tool, not a magic box. It is not perfect, but still provides all the tools to achieve excellent results, it is just important to know how to use them. The gist is that with a little hard work and a few hours of work, the result is this one, above on the right.
Now we have to paint the background, soil, plants, and all accessories. I have created some brushes that have helped me a lot, especially a leaf-shaped one, configured with Scattering, random variations of angle and colour, and other trinkets, and another one made of circles with Scattering, variations in angle, roundness, colour, and more. Brushes are quite a complex matter, and there's nothing like spending a few hours trying out the various parameters and discovering their effects. As a final step, I locked the transparency of the layer with the stroke and I coloured it according to the areas of color with which it came in contact.
With patience and some omnidirectional insult, the result (at the top of the page) really satisfied me.