While it might be difficult to paint concrete objects from scratch on Paint.NET, creating background sceneries is comparatively easy due to the existence of various helpful effects.
Gradient tool (Tools window)
In
Color Mode, the Gradient tool allows you to create a transition between the two colors of your choice. It will start with the primary color and slowly blend into the secondary color in a linear or radial shape amongst others (top bar). If one of the colors is transparent, the other color will slowly fade out instead.
The sky in the following image was drawn with the Paintbrush tool, using different colors with low hardness and opacity and a large brush width, while the sun was created with the Shapes tool.
To add the glaring effect to the sun as seen in the second image, the Gradient tool can be used: set the primary color to white and the secondary color to transparent, and select the radial shape, in order to cause the white color to slowly fade away from a chosen center.
The Gradient tool includes a
Transparency Mode as well (top bar), which causes the entire active layer or selection to fade out, allowing it to be blended with the underlying layer.
Radial blur (top bar > Effects)
This effect blurs the active layer or selection around a chosen center, which is useful for creating blurred sun rays. The Gradient tool in
Transparency Mode was used to let the layer with the rays fade out radially around the sun.
Hue / Saturation adjustment (top bar > Adjustments)
This is probably one of the most useful functions in Paint.NET, as it allows you to edit the colors of the active layer or selection retrospectively. For example, the image above can be adjusted to depict a blue instead of an orange sky.
Twist distortion (top bar > Effects)
The twist distortion is a fun effect that twists the active layer or selection into a spiral around the chosen center. This can be used to create surreal effects for example in the sky, to make the background look more dynamic and interesting.
The sky was drawn with the Paintbrush tool similarly to above, and then edited with the Twist distortion, while the sun in the layer above remained unaffected.
Dents distortion (top bar > Effects)
This effect is meant to distort the active layer or selection as if it was reflected in turbulent water.
The flexible settings of this effect allow it to simulate a wide variety of different shapes, such as vegetation and flames.
The base of the following images was drawn with the Paintbrush tool, using different colors with low opacity, and a large brush width. To create the bushy look of the vegetation, simple clicks were used opposed to strokes in order to get multiple round forms.
After the distortion, details can still be added to the images like the sparks in the flames below