It doesn't matter what I try, I'm not able to set the title color of an inactive window.

Only the foreground window can be changed.

In your guide there is only a ColorizationColorCaption entry.

Am I missing something or what I'm doing wrong?

Edited January 13, 201610 yr by 0xDEADC0DE

There is a small bug in 1.4.5 which prevents setting inactive colors. If you have Windows 10 Version 1511, you can change color in registry using HKCU\SOFTWARE\Microsoft\Windows\DWM - AccentColorInactive.

I've tried that already and the color doesn't change. I've created a REG_DWORD with a value of 0 for black. No luck.