STEPS TO REPRODUCE
1. Add a calendar "Birthdays and Anniversaries".
2. Try to change the color by right-clicking on the entry in the Calendar Manager, then hovering over the submenu "Calendar Colors".
The menu entries "Assign Color..." and "Disable Color" are disabled.
The menu entries "Assign Color..." and "Disable Color" should be enabled.
KDE Neon 5.14
See https://forum.kde.org/viewtopic.php?t=138840, https://forum.kde.org/viewtopic.php?t=139052 (nevermind the topic subjects, these are both concerned with not being able to change the birthday calendar color)
Interestingly, by accident I just now found out that it *is* possible to change the color, via Settings -> Colors and Fonts -> Resources -> Birthdays and Anniversaries.
However, me and the others (see referenced forum posts) are testimony to the fact that this is not very discoverable. Is it not possible to enable the usual context sub-menu?
I agree with Daniel, this should be able to be set as it is for the other calendars, through context menu.
*** This bug has been marked as a duplicate of bug 377154 ***