Hmm, you have 18 colors but only 3 colors on the sky, I'd say reassess your palette usage and make the sky much more appealing. When you have 18 colors, you shouldn't see dithering like that going on in something like the sky that takes up a majority of the canvas. Make that sky much more smoother and appealing and reevaluate the excessive colors in your buildings. You have some pseudo-noise texture going on but in my opinion it just doesn't need it. The buildings would look absolutely fine with a flat single-color texture. The sky thing also applies to the water.