The Yellow Sign
From Tatters of the King

The yellow sign is some sort of occult symbol. Its appearance appears to be related to The King in Yellow. Apparently in Yhtill seeing the yellow sign means that the King in Yellow is about to make an appearance.
In the Play
In the play Carcosa, or The Queen and the Stranger, The Stranger wears the yellow sign, apparently in defiance of the King in Yellow.
Unusual Properties
The appearance of the yellow sign appears to have had a profound effect on some of the viewers of the [play], especially Nathaniel Browne.