I believe this is a bug in MacOS. Reporting in case anyone else encounters as I have found the solution.
I have a list called career progression coloured purple. I updated the colour in Apple Reminders app to green. The same list within good task has had its colours updated, however the tasks within that list remain purple.
Tried moving the tasks to another list and back again doesn't fix it.
Any new tasks added remain purple rather than green.
The solution was to change the colour from within goodtasks.