From "Five Cuts: Key for Dodgers is having Kuo to KO trouble" by Tom Verducci at SI.com:
[...] the Dodgers need Kuo, who was left off the NLDS roster because of a mysterious arm ailment. Kuo's left arm began changing colors when he warmed up to pitch in the final series of the regular season. Though it has been reported that Kuo was suffering from a circulatory problem, the Dodgers ruled out any structural or medical concerns, preferring to think he simply fell victim to getting overly anxious.
His arm STARTED CHANGING COLORS? Um, that can't be good. Let's hope the inclusion of Kuo on the NLCS roster doesn't cause Dodger fans to use language as colorful as his arm.