This is extremely useful to take advantage of (that you can store RGB values in 0-alpha pixels). I've written some pretty simple but powerful shaders for a game I'm working on by utilizing transparent pixels' "extra storage" which allowed for either neat visuals or greatly reduced the number of images required to achieve a certain affect. For instance, I wrote a shader for a characters hair that had source images colorized in pure R, G, and B and then mapped those to a set of three colors defining a "hair color" (e.g. R=dark brown, G=light brown, B=brown). If I didn't have the transparent pixels storing rgb nonzero values, the blending between pixels within the image would jagged and the approach would have been unacceptable for production quality leading to each hair style being exported in each hair color. As a total side note I really enjoyed the markup on the website. Seeing the matrices colored to represent their component color value is really helpful for understanding. Nice job author!