When thinking about the tone you want to convey, don't stop at the first thing you think of, expand on it. What do you mean when you think the place should be "dull"? I'm guessing you don't mean "boring", but something else - desolate, dreary, ruined, empty, melancholy, etc. As I mentioned in my post, you can communicate those without being boring.
Don't stop at adjectives, either. What colours and shapes do you associate with those adjectives? What in the scene could produce those colours and shapes? When thinking about the colours and light, you need to balance plausibility (what light sources and objects make sense in this scene, etc) with artistic needs (what colours and composition would be most effective, etc).
Low contrast: small difference between the colours.
Colour consists of three components: hue (green, red, orange, aquamarine, violet, etc), saturation (how intense the colour is; the lower the saturation, the greyer the colour), and value/brightness (how light or dark the colour is). Contrast is the difference between colours in any of these components.
In your case, the value contrast is the bigger contributor to making the scenes look very busy, there's too much difference between the darks and lights of unimportant elements.
Cool colours: colours are commonly split into groups of "warm" and "cool" colours. Red, orange, yellow, yellow-greens, red-violet are usually considered warm, while blue, blue-green, blue-violet are considered "cool". As the names suggest, these colours suggest physical temperature, but they're also associated with different feelings, especially when combined with saturation. Warm environments usually feel welcoming, while cool environments often feel foreboding or melancholy. Of course, there's a huge range of tone/feeling you can communicate with colour, it's not just a simple spectrum between welcoming and unwelcoming.
Although lower-saturation ("duller") colours help create a feeling of abandonment, dreariness, melancholy, etc, you usually shouldn't go completely grey (0 saturation), because grey has no hue, so you're missing out on subtle mood cues by using it. Having even just a little bit of hue in your "greys" helps a lot. Plus, in real life, objects are almost never pure grey, even "grey" ones.
If you're willing to post a 1:1 image of one of your scenes, I could try to do an edit to show you what I mean more clearly.
