How do pixel boundaries relate to the fact that you cannot avoid color fringing with subpixel AA? You could shift your whole framebuffer (or maybe just a glyph, with possible caveats) by a subpixel or two and have GBR or BRG pixels, it doesn't matter. What matters is that you need to keep the hue constant across triplets of subpixels to avoid fringing, and this is what grayscale does, and what subpixel does not do because it is its whole point. Subpixel basically assumes all your subpixels are the same color.