The master thrall Galt was in control of the collars. When activated, collars of obedience would subject thralls to extreme, possibly lethal, pain. Galt's eyes would glow when he activated the collars. The Providers were also able to control the collars and had the power to deactivate them completely. When completely deactivated, the collars could easily be removed by the thralls who wore them.
The collars differed in color identifying which of the Providers owned the thrall. New thralls had white collars while owned drill thralls had red, green, or yellow collars. Galt, the Master Thrall, wore a multicolored collar of red, green, and yellow, denoting that he was owned by all three. (TOS: "The Gamesters of Triskelion")