The Royal Locks are a hereditary trait of the Royal House of Idris on Nalthis. [1]
Members of the Royal House can using their will power like a muscle change their hair colour or cause their hair to grow spontaneously. Normally the hair reflects their emotional state and changes like these happen naturally and without training. Using the ability, especially to grow the hair takes effort and leads to hunger and exhaustion.[1]
The Royal Locks are evidence that the Royals are originally descended from a Returned.[2]
Only members of the Royal House that are potential heirs to the throne have and can pass on the Royal Locks. [3]
Hair Colour Indicators
Black = Calm or control [4]
Grey = (We are never given an emotion for this one Vivenna uses it as a disguise)[4]
Red = Anger or embarrassment [5]
Yellow = Happiness or excitement [6]
White = Fear or panic [7]
Known Users
Below is a list of those known to have the Royal Locks
Also note that Vasher suggests that all Returned can alter their appearance to be as they believe they should look. This explains Denths addition to this list. Potentially all Returned could use the Royal Locks if they were aware they had the ability. [2]