This is a really tough issue, because text is just as important as graphics. Every game has some text, yet text tends to bias this analysis towards resolution instead of color. Moreover, restricting the data sample to images that contain no text whatsoever would drastically reduce my sample size. I could argue that this deliberate exclusion of a signficant portion of the screens from real-world games biases my results against resolution, but in the end I decided to err in the direction of conservatism. Another potential source of error arises from the compression overhead.
Most compressors have a certain amount of overhead.
If the overhead were a significant factor in this analysis, then the compression factor would increase as the buffer size decreased. This does not appear to be the case. I conclude that overhead is not a significant consideration. We get more bang for our buck, more picture for our pixels, with black and white images than with color images. I think this is because the choices an artist faces with a black and white image are simpler than those with a color image.