Thanks! That pointed me in the correct direction! 
I set my active to : ff767676
and inactive to : ff484a4c
Which is a dark grey inactive and a medium grey for active.
For those who wants to make a custom inactive color as well, this is where you find the registry DWORD: Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\DWM
Just follow the link to the guide in the OP.