Back when I had lots of free time, I went through the usernames that were in different colours to see what they meant. Each day, there would be one name that is red and bolded. It refers to the user who has the most commendations ([Order name] of Avon). Other members who have red usernames belong to the same order as the member who has the 'Of Avon' achievement. Users who belong to other orders all have blue usernames. Hope that helps!
For example, MHD currently has the Marauder of Avon title since she has the most commendations. All the other marauders have red usernames, and everyone else has blue ones.