Text - Negative Color from Background

Hey guys :D I have a little problem with my text. Let's say I have a black text, if there randomly passes a black background then the text won't be visible right ? Because it's black too. So it has to be white to be seen.

Well I want to make something like this but I don't know how. I mean how am I supposed to detect which color is in the background of each letter in the text and then change each letter independently from the whole text ?