Indigo

Day 6 of the spectrum series and a difficult color.

Wiki says:
Indigo is the color on the electromagnetic spectrum between about 420 and 450 nm in wavelength, placing it between blue and violet.

In my quick google search I found no interesting lyrics or quotes so I'll take the liberty of paraphrasing and leave you with:

"Hello. My name is Inigo Montoya. You killed my father. Prepare to die."
~Mandy Patinkin/Princess Bride