Synchronization of Custom Colors
Week Calendar's Custom Colors can be synchronized among different iOS devices by enabling Synchronize Colors.
In Week Calendar go to Settings > General > Synchronize Settings.
Notes
To be able to do this technically, Week Calendar 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).
When you enable ( Settings > General > )Color Syncing, Week Calendar will store the assigned custom color as a small piece of text like [!AB7080] in the notes of an event.
Technical Limitations
Due to technical limitations syncing colors with Outlook is not possible.