How to undo Set Transparent Color after saving the Presentation?

I have been visually comparing two pages of text to verify everything is same except three-characters month abbreviation (at fixed position); in other words, all text except month information is standard.

To do this, I stacked two Full Print Screens. If result is above, then I am confident that two pages are same except for month information. Problem appears when the standard needs to be changed; say Jane is now John.

I need to use this month of Text (which shows John) as standard; to do so, I need to undo the transparent color which is already saved into the Presentation; and delete the out-dated Page (which shows Jane).

How to reset the image for transparent color only? without having to reset other attributes?