Save the edge
When adding color to an object, it’s not always best to fill the entire shape with color. Sometimes the result is more airy, and gives a lighter feeling, if you leave a few millimeters of unpainted paper around the whole, or parts of the object. It is also a way of highlighting what is in front of the background, which is usually the subject itself or at least part of the subject.
This is especially evident when a dark object is placed against a dark background. The same can apply even to relatively light backgrounds. Whether or not to leave a thin unpainted edge around an object depends somewhat on the style of the painting. If the painting is precise and photorealistic, leaving edges does not work well, as that is not how it is in reality. But if the painting is done in a somewhat freer style, it often works better than painting to the edge.
Here below is an example, with a painting of some apples. The first picture shows how I painted it, with highlights on the left side of each stem. The second picture shows how the painting would look if I hadn’t made the highlights. It may be a matter of taste, but I prefer the lighter mood in the painting with the highlights.
The second example is a very loose painting of an onion. In a painting done in a quick, sketchy technique, I think it is natural to leave some highlights. All the white, quickly painted highlights around the root threads and the onion’s shoots contribute to the feeling of dry onion skin. In the second picture, I have removed the highlights, making the image feel heavier and losing its lightness.
Here below are some details from larger paintings that include people. When I paint people in urban environments, I almost always leave a white edge around them. The woman on the bicycle has highlights, especially around her hair and arms, where the background is busier and darker. It is a way to separate the background from the subject. All the people in the background also have highlights around the edges, as do windows, doors, and other details further back. Without these white edges, everything would become a massive jumble of dark colors, making it hard to understand.
The second picture shows a couple walking on Kiliansgatan in Lund. It is a fairly freely painted watercolor with highlights around the couple in the foreground, as well as around poles and window panes, and the top of the fence.
Since I often paint scenes from Lund, there are many windows in my paintings. When a window is an important part of a painting, I always leave a small gap between the glass pane and the window frame and mullions, at least when the panes are quite dark. This is a way to lighten the impression that might otherwise feel heavy. In the detail from a larger painting below, you can see this; you can also notice that the fascia board on the right has a narrow highlight along the edge.
In the second example above, also a detail from a painting, it shows how the dock’s posts become distinct with the help of small highlights. They would have been very difficult to see without the narrow unpainted edge around them. When something dark in the image stands against a dark background, you can use highlights to make the subject clearer.
In summary, it can be a good idea to sometimes leave a bit of unpainted paper around objects, especially when the background is dark or cluttered. This helps the foreground object stand out clearly and not blend into the background. A narrow unpainted edge also helps create a lighter impression in a painting that might otherwise feel heavy or overloaded. So, even though it’s not truthful to reality (because things never look like that in real life), it serves a purpose: to make a subject easier to understand and lighter in appearance.