Hello, the effect you’re looking at in the example picture seems like a screen tone. There’s a method to set up a screen tone layer which you can paint over in this thread Gradient Tone Tool.
There’s almost certainly a way to make a screen tone brush, if someone hasn’t already. You would need to upload or use a screen tone texture to the Pattern section of the brush settings and make sure the anti-aliasing in Brush Tip setting is off. Making sure opacity and Strength dynamics are off will make sure it’s a solid colour.