@R4ykp 

You can add a single unit. The process is described here: How to add an additional unit to your Deco network

I don't think there are Deco models with color other than white. If you want Deco model that does not stand out, consider M5, M9 Plus and X50-poe. Their form factor allows them better blend into an environment.

You can also wall or ceiling mount these Deco. For M5 and M9 Plus you'll need third party wall mount holder (they are different for these two models, so choose the right one), X50-poe comes with everything necessary.