Pernosco can show a view of the application windows' contents changing over time, as a series of screenshots. You can compare any two screenshots to see the differences between them, and zoom in to the pixel level and inspect the color values of each pixel. Clicking on a pixel explains the dataflow leading to that pixel.
Try out the demo for yourself!
Currently we support screenshots for applications that use X11 and draw to their windows using only simple bitmap copies. (Typical modern applications, like Firefox and Chrome, fit this model because the X server's complex drawing operations don't meet their rendering needs.)