Here is how I create one of the simple textures I use in my digital paintings using Infinite Painter.
Using the Castell brush with the brush Size set to 10 along with the Rectangle shape tool I drew a yellow square. Then with the Fill tool selected I filled the square with the same yellow color.
Using the Old Oil paint brush and the same yellow fill color I drew over the yellow square with random movements of the brush. This brush tool doesn’t paint a solid color but rather a streaked effect of subtle light yellow and grey shades. The streaks are most pronounced when the brush strokes begin. However the longer the brush goes over the color without lifting the brush from the screen the streaks fade and blend with the solid yellow. You can get some random effects that won’t appear until the area is filled with another color.
Finally the yellow square is filled with an orange color. The Gradient Tolerance of the Fill tool is set to 2 or 3 so the color fills the shape leaving some of the streaks showing with variant shades of yellow without filling it solid.