Jonquil is a shade of yellow, first recorded in English in 1789.
In The King In Yellow, jonquils (the flower) are mentioned once, in passing, in the story In the Court of the Dragon
Jonquil is also an entry in Fantastic Worlds and possibly the name of the broken doll described there as communicating with Cordelia, and the title of a story, also featuring a doll of that name.
Jonquil was the suitably decadent name given by Oscar Wilde to his lover, Bosie.
By coincidence, Jonquil was also the name of the wife of Fritz Leiber, a writing colleague of Lovecraft's.