Is there a way to change the color of the navigation bar? Or to make it transparent Some apps change the color of the navigation bar color to their color scheme (or even make it transparent). But most of the time it remains black. Is there a way to change its color for all screens or to make transparent?

It's okay if rooting or custom ROM is needed for this.
Install GravityBox compatible with your Android version. It requires a rooted device with a compatible Xposed Framework installed. Once you install it, enable its module in Xposed Installer app, launch it, go to Navigation tweaks, and make changes which you are looking for. Master toggle listed there requires a reboot for changes to take effect.