Why is there only one hilighter shade on black and white ereaders?

I plan on getting a Kobo Clara BW soon, but I've always been curious why they don't let you higlight in different shades of gray, because it should definitely be possible. You can look at a black and white comic book, they use a buch of different shades of gray to shade different things, so why not use it in an ereader? I'd assume that unless it's just too hard to tell the difference, in which case it could be a beta feature, it's probably just to get you to spend the extra money on a color ereader.

Does anyone know any ways that let you download extra highlighter shades? It seems like it might be a KOReader feature.