There is no way to color your highlights on the E-Ink devices. The best that could be done is as you said, using shades of grayscale -- but Amazon did not see fit to chase down that possibility.
No, nobody has rewritten the framework to add that feature in.
As for viewing all highlights, you can see it on
https://kindle.amazon.com/your_highlights
Or look at the My Clippings.txt file on your Kindle, which saves a copy of all your highlights/notes/bookmarks.