, I have managed to alter (to some extend) that color you was talking about.
I think it is called Color of Selected Items
First step: Close all Office applications.
Change standard color of Selected Items through
Personalize->Window Color and Appearance->Advanced settings
Open Regedit - Navigate to your user settings
And change value of MTTA and MTTF to zeros.
Same thing could be done for Word and others.
When Outlook will open next time it will copy Selected items color to those parameters in RGB format.
However, this color will be used with some form of transparency. And looks like that transparency is hard-coded somewhere and cannot be easily changed.
So, color will be the same as Selected Items, but always with some sort of transparency.