Tips on creating a suitable color scheme/texture set? (CSGO)

I know you already touched upon this, but if you don't have any references, it's really hard to get an idea of what to put down in the environment. It seems like this is part of what you're experiencing here. Spend a little time assembling some references that fit your vision for the environment. It makes your job a lot easier and will certainly give you some ideas when you're feeling stuck. You don't have to find pictures that mimic what you're constructing (which obviously might be difficult or impossible in this case) but just things that seem inspirational or give a rough idea of what the level should 'feel' like.

I think when it comes to areas, you need some more geometry to help you along, as you've mentioned. This in itself helps some areas feel more realistic and less 'off'. Think about how structures are constructed and look at your references. There are often trims, ledges, or variations in geometry at the top of structures or where they change to a different material. When poured concrete is used for surfaces like walls, there are often reinforcing structures or trusses left in place, or at least marks from where they were installed during construction - the texture on the lower curved portion where the window is looks like a floor texture because it's just one continuous, concrete surface, for example - it's hard to see for certain because I had to zoom in a little on the image, but just making an example of something that looks a bit 'off' here

Not to put too fine a point on it, but I think the issue here is that you're expecting textures to help fill in a blank resulting from your geometry. I'll use this image to illustrate what I mean.

(The image isn't mine, it's from the HAER: http://www.loc.gov/pictures/collection/hh/)

A pretty boring structure (it's from a military base, no architectural creativity here), but I think it serves as a fitting example. You can see that it's constructed almost entirely of one material; a flat plaster or concrete of some sort, but it still has some interest because of the construction and the lighting (the construction being extremely simple, too). The point I'm getting at by all of this is that the materials really work in conjunction with their underlying surfaces and how they were built and especially with the way the environment is lit. You don't need a huge set of unique materials to make an environment feel convincing, sometimes less is more.

I'd try adding more geometry first and see how things are feeling then, but when it comes time to create materials you really need some reference...ehm, materials to work from. Even a handful of photos or a concept paintover or sketch will do the trick. If I'm really focused on working around a colour palette I'll generate one based on the key colours I have in mind or from the references (just some swatches in Photoshop) then work on adjusting their colours to match the swatches during the process of actually creating the individual materials. I'm no pro texture artist though, I'm sure someone else here could give you a better insight into this process