[calsify] non-CSS-named colors for events

[Ricardo Signes <[e-mail]>]

Hello!

tl;dr:  I think the rarely-used COLOR should have a facility for RGB colors, not just named colors.

RFC 7986 <https://datatracker.ietf.org/doc/html/rfc7986> provides the COLOR property <https://datatracker.ietf.org/doc/html/rfc7986#section-5.9> for specifying color on a per-event basis.  It defines the value as a string that must be a named CSS3 color.

Calendar users, unsurprisingly, very often push for more control, and want a full selection of RGB colors.  This is permitted in jsCalendar, where the color <https://datatracker.ietf.org/doc/html/draft-ietf-calext-jscalendar/#section-4.2.11> event property can be a named color *or* an RGB hex value.  At present, there is no means to map a non-CSS-named color from jsCalendar back to iCalendar.

Erratum 5449 <https://www.rfc-editor.org/errata/eid5449> was filed, suggesting that RFC 7986's COLOR property should have allowed for "rgb(r,g,b)" formatted hex colors.  This erratum was not accepted.

We (Fastmail) plan to support RGB color for events using JMAP for Calendars and jsCalendar, and so need to map these values back to iCalendar.  I am not familiar with any existing clients that use the COLOR property, but would be happy to be pointed at some, and at how they may handle this problem.  In the meantime, here are options we have been thinking about:

 1. act like the erratum was accepted and use "rgb(r,g,b)"; maybe somebody else has done the same, but parsing that, especially given the structural commas, is not appealing
 2. just put #RRGGBB into COLOR as needed; any human would understand what is happening and could adjust accordingly; file a proposed update;  nice and simple, directly maps to jsCalendar
 3. like the above, but add an attribute to the property:  `COLOR;VALUE=HEX:FF0000`
 4. store weird colors in some experimental property; this is unappealing, as it seems like a bit of a dead end
 5. store weird colors in CATEGORIES, which Fantastical does, but is otherwise as unappealing as using an experimental property
-- 
rjbs