Imagine you want to paint an orange or similar object that has a textured surface one way of doing it is to paint the orange tonally so it looks three dimensional. Cut a paper mask the same shape as the orange and place this over the orange so only the orange is visible. Whilst the paint is still wet, dip a brush into clean water and splatter the painting. If this is done carefully the water dilutes the paint in small random shapes and as it dries these shapes take on a very delicate outline, almost perfectly depicting the texture of the surface of the orange.
Another method is to use wax resist. Here you can use a candle or oil based crayons both of which resist water. Draw grasses for instance and then overlay a coloured wash. The results are very random but can create some very interesting effects. Imagine a field of corn; sketch the corn with the vigorous marks of a wax candle and overlay with an ochre wash. By adding further layers and varying the tone and colour of the washes you can create marvellous results.
The use of salt is another way of creating random textures. Apply salt to a wet wash. It creates a delicate crystalline pattern, but this depends very much on the size and type of salt crystals, how much you use and how wet the wash is. You will need to experiment, but the effect is great for starry skies, the sparkle of sun on water or the pebble dash on a building.
Of course, there is always the splatter method using an old toothbrush. Mask the area to be splatter because the splatter can get everywhere. Mix up a wash of colour and dip the old toothbrush in it. Draw a hard object, like a knife, across the surface to the brush to create the effect. Experiment with holding the brush at different angles and distances from the paper.