Take a look here for some more information about the challenge events. For instance, you will see that each different challenge has a specific reflected color - there’s one challenge for each color. They are monthly, so get used to them. And they are called challenge events for a reason - they are a great source of valuable items, once you are able to complete them.
(Note: all of the challenge events are being “remodeled.” Avalon was the first to be updated - the information on that link will be updated for each future event as it becomes available.)