Victor is a concert violinist. Depending on the piece he's playing, he tells a friend, the music might be purple or it might be pale green with pink and orange edges. Victor is displaying signs of

Victor is displaying signs of Synesthesia

Explanation:

Synesthesia is a condition in which someone's sense (for example, hearing) is  perceived at the same time, as if by one or more additional senses such as sight.

Chromesthesia or sound-to-color synesthesia

Explanation:

Chromesthesia or sound-to-color synesthesia is a type of synesthesia in which sound involuntarily evokes an experience of color. sounds inducing color while music is been played. The Individuals with sound-color synesthesia are consciously aware of their synesthetic color associations in their daily life. They perceive color while listening to music and experience the colors in addition to the normal auditory sensations.