When there are multiple events for a given date, they are sorted by the title of the EventList or Timeline that defines them and the date color is determined by the first event in the resulting list.
Here's the code from the showDay() macro that does the work:
<$wikify name="colors" text="<$list filter=<<todays_events>>>{{{ [<currentTiddler>split[;]nth[1]get[eventcolor]] }}} </$list>">
<$set name="eventsbg" filter="[enlist<colors>first[]addprefix[background:]addsuffix[;]else<eventsbg>]">
If you have a Timeline named "Absences" (using the OldLace color) and an EventList named "Meetings" (using the LightGreen color), then the OldLace color will be applied, since "Absences" comes before "Meetings", alphabetically.
The only way to influence the color that is used is to rename the Timeline or the EventList so that the event color that you want to have priority occurs first in the sorted list of events. For example, you could rename "Meetings" to "1Meetings" so it comes before "Absences", or rename "Absences" to "ZAbsences" so it comes after "Meetings".
However, renaming a Timeline can be quite a nuisance, because the name of the Timeline is also used to tag each individual tiddler that belongs to that Timeline. Thus, you would have to change not only the name of the "Absences" Timeline tiddler itself, but also update the corresponding "Absences" tag on every individual tiddler belonging to that Timeline.
Fortunately, renaming an EventList is much easier than renaming a Timeline, since there is only one tiddler involved with no extra tagging issues. Note that even if you rename the "Meetings" EventList tiddler to "1Meetings", you can also give that tiddler a caption of "Meetings", so that the leading "1" won't appear in the Calendar interface.
When I wrote this part of the Calendar, I had considered adding extra code to address this issue by allowing explicit specification of the color order, but all the implementations I could think of would have added considerable complexity and processing overhead to the Calendar rendering logic.
I hope this explanation isn't too confusing. Let me know how it goes...
-e