You can use a wash to cover a large area, but also do small very local things like picking out individual bricks in a brick wall. Doing this randomly enough to get a natural look can be surprisingly hard! It usually takes a bit of time to get into the right state of mind and then it just goes automatically. I tend to go ovwer the whole wall, sporadically picking out bricks, then getting denser and denser. Meanwhile the wash is changing a bit as I add paint and/or terpentine, so at times you’ll darken bricks a lot, then after the wash waters down not so much anymore. This gives some nice variation, but will not look good if you systematically work your way over a wall from left to right. Of course you can do the same with light colours as well, or rooftiles etc…