(note I haven't worked on subpixel AA for font rendering, but have for something different) the amount of color you apply to each subpixel has different levels

(note I haven't worked on subpixel AA for font rendering, but have for something different) the amount of color you apply to each subpixel has different levels of intensity depending on how deeply into the pixel the shape goes, so you would need to blend with whatever is behind/in front of it. Also you can't just do something like write (128,0,0) to a pixel and leave it at that, you'll have a red pixel. It would need to be blended with what's already there.