Pixeditor

All objects have prescense in three dimensions. Their form becomes powerfully defined under a light source. Planes facing towards the source are illuminated. Planes facing away are starved of light and remain dim. On a two-dimensional computer screen, it is the job of this shading to convey a sense of form and depth.

Now that we have shape from our lineart, and the foundations of colour, we can variate that colour to really bring our piece to life.

Establishing Light Sources

In order to begin shading an object, it is important to first establish where light falling upon it is coming from.
For outdoor settings, or indoor areas with consistent overhead lighting, it helps to pick a constant direction for light to fall from. Some people like their light to fall from the upper left corner of their image - I prefer the upper right and will use this for the rest of this tutorial.
This common kind of light all strikes your object at the same angle.

Areas with one or more lightsources illuminating shapes all around them are a special case and requires a little more work and attention. Good use of alternate light sources helps create mood and atmosphere in a scene. It is a rather obvious point, but important to note that in these cases, light noticeably decreases in intensity the further from the source you are.