Hitting some bricks with a dull satin will darken them and change sheen, which can also vary.
Modern bricks may even be sealed with a gloss.
New to washes? Try cheap water based craft acrylic from companies like Apple Barrel (wallymart, etc) If you don't like it, water will remove it. Soap and water if it cures a day or two, usually with a scrub brush after that.. Getting it all off seldom matters with washes anyhow. Also great to practice dry brushing with it. Only 80 cent for a 2 once bottle. Lots of color variations and easy to blend. I stagger my colors giving a day or two between coats, so if I do wash a layer off, the one under it remains solid. The only drawback is it is thick and has more texture when dry compared to smooth lacquer or enamel. It also is softer by far once cured. Clearcoats can help protect it.
Use black, brown, red, green, and grey for rain grime. Basically a few pigments of everything with a heavier base in black or brown is what you see outside. Colors in nature are seldom pure anything.