How to remove colored window outline in Windows 10?

If coming here due to a blue box appearing around anything that you select (i.e. mouse click or tab to):

This is due to Narrator running.

To turn it off hold down the Caps Lock key and press the Esc key.

You can also permanently disable Narrator.

From a Reddit thread:

1. Hit WIN+R , type Control Color and press enter.
2. Lower "Color Intensity" to 0 (All the way to the left. This does not affect the accent color intensity whatsoever)
3. Feel at peace with your desktop again

1. Open regedit and navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Accent
2. Set the value AccentColorMenu to 0.

Take a look at my answer to the question How to remove accent color from Start menu tiles only for more info.