Originally Posted by shev

"Mr. " prefix is an identifier of the developer user group, same as coloured names for Admins/Smods/Squads etc. Developers who also hold other staff positions do not have the prefix.

You cannot change your username colour or add a custom prefix like "Mr. " through Profile Customization Store. However, you are able to add a one-character prefix with VIP subscription that can be bought here.