Cast member told us that because DW did not have her own MDE account, but rather listed as F&F on mine, that was the reason hers did not light up purple... much to her dismay, as purple is her fave colour!
I followed the link erionm posted above and it seems to confirm this in this passage:
With that in mind, I’m pleased to share that members who have affiliated their My Disney Experience account with their Disney Vacation Club Membership will now see their MagicBands trigger a unique purple-to-green color sequence as they enter Walt Disney World Resort theme parks, Water Parks, DisneyQuest or the ESPN Wide World of Sports Complex.