Dead pixel with but with inverted color in rMBP
Dead pixels aren't necessarily "black pixels". Although it's common that the dead pixel stays black, they also come in different colors and, as in your case, they can change colors too.

The way to tell if it's a software problem is to check if the pixel appears dead right after turning on the machine. If that is the case, it must be a hardware problem because the software has not been loaded yet at this point.

Check this, and if it's a hardware problem, my guess is a failing graphics card or display. Time to take it to an Apple Store :(