On Ubuntu 18.04 (running under VirtualBox), icons in the indicator bar change colour based on the state of the current window.
In the screenshot below, I have have a terminal window maximised and the icons are all the same colour. The icons to the right are the default icons and the icon to the left belongs to one of my indicators. The icon/colour scheme which I have used is ubuntu-mono-dark, which is what I believe the default icons are also using.
If I unmaximise the window, the default icons change colour:
To further muddy the waters, if I unmaximise the window and dock it with the top bar, the icons revert back to the same colour as my icon. If the left navigation bar (dock) is set to never autohide and I dock the unmaximised window against the left navigation bar, again the icons revert back to the same colour as my icon.
Although I have determined from gsettings that the default theme is Ambiance, I cannot find the corresponding icon set for that theme (that is, I cannot see how/where there is a link between the theme of Ambiance and the icon set in use which is ubuntu-mono-dark).
Is the underlying mechanism such that when a window is unmaximised, a different theme or set of icons are selected?
If I create icons for that theme (place icons into the appropriate /usr/share/icons directory), will my icon will magically switch back and forth?
EDIT: Added screenshot of gnome tweaks tool running inside the VM: