Above a certain temperature the vertical mast of a thermograph is black, which indicates that a local play will normally entail a loss in that range of ambient temperatures. However, the mast may also be colored.[1] A blue color indicates that Black may make a local play without loss, a red color indicates that White may make a local play without loss, and a purple color indicates that either player may do so.
The most common colored mast occurs with (theoretical) sente positions. Just above the base of a sente mast the mast has the color of the sente player. That coloring indicates that the sente player has the privilege of making the play with sente in that temperature range (as a rule).
Colored masts are related to ambiguous positions, that are not clearly either sente or gote. Just below the base of the mast of a sente position the wall of the sente player is vertical, which indicates that the opponent will reply to the sente play. Some ambiguous positions have such a vertical wall, but the mast just above the base is black or purple, so that there is no privilege. On the other hand, the mast of some ambiguous positions have a colored mast that indicates privilege, but just below the base of the mast each wall angles in the direction of the player, which indicates that the opponent does not reply to a local play (gote).
Purple masts normally indicate miai positions, where either player can play, but gains nothing, on average, from doing so. (Inclined sections of a ko mast are purple, as well.) Such vertical purple masts indicate double ko threats, where either player may make a local play as a ko threat or to take away the opponent's threat.
[1] The idea of colored masts in thermography was introduced by Bill Spight.