The make-up guys say it's "hard to describe"
Quoting this bit of Make-Up Artist Magazine:
But designing the crimson-hued character proved challenging for make-up department head Jeremy Woodhead and Nik Williams from Animated Extras. As Woodhead explains, “The red color was actually the hardest thing to figure out, because we didn’t want him to be a bright scarlet, which would look slightly absurd, so we ended up with a color that’s hard to describe,” Woodhead told the magazine. “In some light, it looked pink and in others, red; it was like a red cabbage or beetroot color, a purple-pinky red.”
Joss Whedon wanted Vision "human-colored"
Info taken from Entertainment Weekly; arguably, red is closer to human skin than purple:
For Vision, played by Paul Bettany in Avengers: Age of Ultron, Meinerding [Marvel's head of visual development] remembers drafting several possible looks for the android, including one with a gold faceplate and graying body. That design nearly became the character’s final look before the producers settled on a more classic appearance.
“[Director] Joss [Whedon] really wanted Vision to stay human-colored,” Meinerding says. “He really wanted him to look like Paul Bettany with the bit of styling on him, and when we did versions of that, it seemed hard to make that look cool and interesting.”