Premultiplied alpha has some further advantages: 1. Bilinear filtering is always rendered correctly. Without premultiplied values, there are some strange cases: When a fully transparent pixel has a black rgb-color, the gradient to a visible pixel gets darker in the middle, in a similar way the gradient gets lighter when the rgb-color is white. You have to carefully craft the texture, when not premultiplying the texture. 2. You get the ability to blend textures additively in the same rendering pass. Flares, fire and holograms need to be rendered with an additive blend mode. With premultiplied alpha, you can simulate/enable additive blend mode when using low alpha values and higher color values. Low alpha values let almost all light pass through the texture. The color values do not get multiplied again, and so are just added on to the background. For this you might only ignore/discard fully transparent pixels (alpha==0). The alpha value of the pixels is 1, on the 0..255 scale. When the resulting color exceeds 100% it is clamped to 100%.
Learning opengl right now and this video came in handy. Also, near the beginning you say that 75% of 40 is 10% but I think you meant 75% of 40 is 30 and we are left with 10% of light passing through.
