Iso refers to the sensitivity of what?

The image sensor (I did the test and I got it right)

Explanation:

ISO originally referred to the sensitivity of film—it's "light gathering" ability. ... For digital photography, ISO refers to the sensitivity—the signal gain—of the camera's sensor. The ISO setting is one of three elements used to control exposure; the other two are f/stop and shutter speed.

a dark purple with a little bright violet red

pitch- the definition of pitch is as follows:

"the quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone."