Synchronization of Custom Colors
To sync colors among iOS devices enable:
Settings > General > Color Syncing
To be able to do this technically, WeekCal needs to be able to store the color somewhere in the event that will get synced to your other device(s).
The only place available for this is the notes field of an event (since there are no free database fields available).
WeekCal will store the assigned custom color as a small piece of text like [!AB7080] in the notes of an event.
Due to technical limitations syncing colors with Outlook is not possible.