Does this mean what I think ?
If a color is set on the device through the calendar app, then you will upload it to the server with the tag COLOR: ?
Or, if the tag COLOR: is set on the server, then it will arrive in the calendar ?
In this case, I use Informant that can set a per-event color, this color is handled by other applications (eg : Pure Grid widget, Google Agenda, Wear apps…)
On desktop side, I know that Thunderbird doesn’t set this value (instead if uses the CATEGORIES field, and categories are matched locally to a color).
Outlook CaldavSynchronizer doesn’t neither, but maybe it can be set through the manual mapping of extended Outlook fields.