So, what is Active, Modified, Selected and Unselected?
Active seems to be the tab with the cursor.
Modified is for modified tabs - overrides only the text color.
Selected Seems to be all tabs that do not have the cursor.
Unselected Doesn't seem to be used that I can find. 

WRT "APPPLY": 
Actually, "Selected" color changes happen immediately - no need to hit Apply.
"Modified" changes don't take effect even when you hit apply.
BTW, for Modified it would be nice if it had a "Sample Text" box like the others - really it should have two (active and selected)
It seems to be a PITA to find a color for Modified that works with the background for active and selected tabs.
The default, yellow on grey, isn't very visible on my monitor.
Even better of course, is to replace "Sample Code" in these cases with "Samples Tabs".