Colours:
I think your colours are good for the most part! The blue pipes in the lower right stand out a bit too much so I would make them a little darker and closer in hue to the purples that you're using for most of the foreground. As long as they're bluer and less saturated, they'll still feel blue/grey even if they're actually purple.
The background hills have almost the same value as the buildings in front of them, so they blend together. And it looks weird for a more distant object (the hills) to be less blue/atmosphere-coloured than objects in front of them, especially when the buildings are established by the foreground to not be blue themselves.
The smoker's colours don't fit with the rest. Their colours feel dull and characterless in comparison to the lovely palette of the rest. Let this purple-blue atmosphere affect their colours, don't use their "true colours".
Readability:
The shadows inside of the building are a bit hard to see at 1x, so I would make them a little bit darker. They help a lot with making the building look occupied, but at first glance, I didn't even notice them. I'd also add some to the other buildings, unless they're meant to be empty.
As mentioned above, the distant hills don't blend into the distant buildings.
I don't understand what's going on with the chimney highlights.
Or those greyish highlights on the building bricks, for that matter.
Misc:
Human brains key in on perfect symmetry even more than they do on regular repeating patterns, so those mirrored "grime" bricks really stand out. I'd recommend repeating them without flipping them.
In addition, I'd recommend another variant or two, or at least switching up where you place the tiles. It looks very strange that all the purple-grime and all the blue-grime align vertically, and that although you have two different purple-grime patterns, one is always on the left and one is always on the right.
Put some grime in those clean areas between the floors, too! The grime on your buildings is too orderly :] Also, you can use grime to help tell a story. Who is the grossest tenant? Who keeps their area spotless, even outside? Who has bluegreen gunk leaking down from their window from the fertilizer they put in their houseplants, and who has redpurple grime coming up from their window from doing a lot of greasy cooking?
Lastly, I'd add some cloudy texture to the sky. That overly smooth gradient stands out a bit much compared with the sharpness of all the other shapes. Unless it's meant to be smog, which tends to be rather featureless, and for which a gradient tends to work well. You could still stylise the gradient, depending on what look you want.
