Actually Category and Colors are two different properties in the CALDAV definition. Thunderbird only supports Categories (and associates a locally significant color to it).
All other networked Calendars I know of only support Colors.
Which makes the two incompatible.
Thunderbird must rethink it's usage of Categories and Colors in order to offer a consistent experience to users who use synchronized calendars between their different devices
2 Years later and its 2025 and Thunderbird cannot implement a simple feature of assigning colours to event entries when creating them or editing them. I understand importing colours from Googles calendar when importing is an issue but not giving this feature within Thunderbird itself is ridiculous.