Why 'six' colors for the chosen set

[ryanwwest]

This document specifies a closed set of six colours chosen because of their extensive support in well-known reading systems. However, most existing reading apps offer a smaller set to their users.

I am not yet very familiar with this spec - however, I regularly use some very popular open-source reading apps that offer more than six highlight colors:

Zotero: 8 colors (technically supports all colors) - by the way, they use the Web Annotation Data Model under the hood:

KOReader: 9 colors

Various readers also support different styles - e.g. Zotero supports full height highlight or underline, and KOReader supports these but additionally strike-through and an 'inverted' style which works well for e-paper displays. For styles, of course every reader will have different offerings and standardization is the goal, but I wonder if there could be some defined concept of style, e.g., similar closed set of 4-6 styles?

[danielweck]

Thorium Desktop Reader currently implements support for the named colour set defined by the Readium specification, which hopefully one day will be aligned with an updated revision of the W3C recommendation. Meanwhile, I too have been wondering about whether the colour set should cover more hues. Thank you for bringing up concrete examples.

As for highlighting styles, Thorium Desktop Reader implements solid background, underline, strike-through, outline. There is no "inverted background / foreground" option.

[HadrienGardeur]

You might want to read the following discussion: #7

TL;DR would be: having more colors is not necessarily a good thing for interoperability across reading systems

[ryanwwest]

Another example with 10 colors and 2 styles (highlight+underline), closed source: https://www.churchofjesuschrist.org/study/scriptures/nt/matt/1?lang=eng (need a free account to annotate):

[ryanwwest]

While I agree with Hadrien that a dozen or more colors gets too unwieldy, I would personally prefer up to 8 or 9 as there may be use cases that require more than 6 colors and I don't think 2-3 more makes them too hard to distinguish (perhaps there is some research on how many highly-distinct colors the average colorblind person can distinguish).

I understand from the linked discussion that more colors would result in more 3-color apps having no colors in common with other 3-color apps, but that also seems to be an issue with 6 colors so some sort of translation might be needed anyway.