Greebling is the process where small detail is added to make something look more complex. This added complexity can make an object appear more interesting, but it also tends to make things look cluttered. A room full of small details looks crowded. A sparse room with one highly detailed wall (a map of the world for instance) can look quite impressive. Painting is occasionally used to provide this kind of interesting detail.
Most house walls benefit from a single, uniform wall colour; we usually decorate with paintings and other objects that provide the details. But this can vary. Sometimes the walls have simple patterns painted on. More often they use a technique to produce a semi-random texture. Some popular options are.
A popular technique. When a wall has already been painted you can add textured patterns by dipping a scrunched up wet rag into a different colour paint and pressing it against the wall. Many colour combinations work well, but plan in advance.
Dry Bristle Brush
Paint a wall a dark colour. Then, when it is dry, paint over it with a light version of the same colour. While the second layer is still wet you can create a pattern with a dry brush. Vertical stripes are particularly effective.
Paint a wall a light colour. Then fix rubber bands over a paint roller and apply a darker version of the same colour. The result looks very thick and textured.
Applying a different layer of paint with a sponge, either a complementary colour or a darker version of the same colour, will produce a textured finish. Use the sponge to dab the paint on.
Apply some gladwrap when a painted wall is still wet. Then peel it off before the paint is dry. This will create patterns in the paint.
An object that makes a good silhouette (a leaf, bird, star…etc.) can be printed on the wall. Strong contrasting colours look best.