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I actually don't accept the premise here, or at least it's not obviously true to me. Specifically, I'm not sure there is that much of a benefit to implementing "bookmarking" at all. My immediate retort is that your browser (assuming it's any of the major ones) already implements bookmarking and has ways of organizing those bookmarks (and likely can support multi-colored bookmarks via an extension).

StackExchange supports bookmarks, but I basically never used it. My main use-case for it (which was clearly not the intended use-case) was keeping track of questions I had some interest in answering but didn't expect to answer in the near future. (Similar to your third use-case.) Even then, most of the time I just never looked at the list of bookmarked questions and so it really just was questions I wasn't going to get back to.

Bookmarks on StackExchange are public which I think is non-obvious and surprising to many SE users. It's also the wrong thing for many use-cases, e.g. every single one you list. Bookmarking (especially back when it was called "Favoriting") implies an endorsement of the question or, at least, some of its answers that is just not there for these use-cases.

So I assume you're imagining a private bookmarking system with maybe the option of making some public. If StackExchange bookmarks were private, and I used it for some of the purposes you list, I'd now be in the awkward position of needing to log into StackExchange just to get access to that information. Also, the bookmarks being private removes the necessity for a third-party to host this data.

I think public bookmarks a la StackExchange are just a mistake. I think using the browser's built-in bookmarking features (or some other [bookmarking solution](https://blog.elink.io/bookmarking-tools/)) is a strictly better solution than adding private bookmarks as a feature to the platform. The color coding you suggest here is just the beginning of a rabbit hole of embellishments on bookmarking that could be added. As the previous link demonstrates, there are entire applications built solely for organizing bookmarks.