The Wearers of Purple are the only ones that wear purple robes. The first part that talks about the Wearers of Purple says:

>  Leaders in the cult are known as Wearers of Purple, and they outrank normal cultists, but no formal grades exist _within_ the Wearers of Purple.  
--Hoard of the Dragon Queen pg 7

The sidebar in the second episode talks about a heirarchy in the _rest_ of the cult, and names the Wearers of Purple.

> There are only five wyrmspeakers, one per color of chromatic dragon. The current wyrmspeakers are Severin (red), Galvan (blue), Neronvain (green), Rezmir (black), and 
Varram (white).   
 
>Most of the cult’s operations are overseen by local leaders called Wearers of Purple for the ceremonial robes that they wear. All of the wyrmspeakers are Wearers of Purple, as are lower-ranking cultists appointed by the wyrmspeakers to preside over cult enclaves.  
--Hoard of the Dragon Queen pg. 19

So, Wearers of Purple are only the five wyrmspeakers, and any local cultist leader, such as Mondath, that you happen to come across.