Change outline colour during windows spread

I don't think it's supported by Unity/Compiz.
It if it supported by it, we shall considered it, of course.
However if it's not there is nothing we can do.

It seems this is baked into Unity and it's not changeable.
There is nothing we can do about this regrettably.

Has anybody inspected the code yet and could tell me where this color is hardcoded? (package, file, line(s))

It is not hardcoded, at least not in 14.04, but theme dependent.
It's this setting:
-UnityDecoration-glow-color: rgb (221, 72, 20);
In this file:
/usr/share/themes/Ambiance/gtk-3.0/apps/unity.css

See:
https://askubuntu.com/a/514084/11317

Thanks for the heads up.

We only checked the gsettings interface for a suitable option, but there wasn't any. It didn't occour that it might be in the theme files. However, Unity Tweak tool is just a fancy user interface for gsettings stuff, and Anything else is basically out of scope. The theme files can be directly edited, but I'm afraid editing system files will get us kicked out of repo, and isn't something I particularly like.

Creating a theme by the same name as current theme in ~/.themes and editing CSS there should do the trick, but whenever theme is changed, this will have to be done again. And since themes can be changed using various tools and we don't have any control over them, the workaround would be a bad idea.

Sorry, this can't be implemented in Unity Tweak Tool for the above mentioned reasons.