"..studies to date have reported that synesthetes and non-synesthetes alike associate high pitched sounds with lighter or brighter colors and low pitched sounds with darker colors, indicating that a common mechanism may underlie those associations in normal adult brains"
The article its-self is rad in that the circle of fifths is very much a "relative" thing to begin with so we are actually seeing some pretty "flavorful" results- but the search for the "true color of sound" is tough when light is 430-730 trillion hz and audio is 20-20 thousand hz. The factor that you then are dividing or transposing by is arbitrary.
I'm not sure what you mean.. Is this a pun?
The circle of fifths is right on the intersection of music and physics; each perfect fifth is (very close to) a 3:2 ratio on the previous frequency, which is probably why pentatonic scales are found across cultures. I don't know how much less "relative" you can get with something so subjective as music.