Here's the difference between synesthesia perceptions of odd and even numbers; the higher the bar, the more people tended to perceive even numbers as that color; the lower the bar, the more people tended to perceive odd numbers as that color
And while we're at it, odd numbers are "hot" and even numbers are "cold". Which somewhat corresponds with the chosen colors.
Soundwise, odd harmonics are perceived as aggressive and gritty, while even harmonics are "round and subtle".