Hi @Clair_Arthur, I’m sorry to hear that your team has been running into issues. We’ve recently make some updates on the My Tasks view to make it more similar to the existing experience in project views. One of these updates added more color options to My Tasks calendar view.
Can you please ask the users impacted to click the “Options” button at the top right corner of their view and select “Asana Default”? This should bring the colors back. 
You can find more information about this launch in this announcement.