Interestingly for patients on pembrolizumab, vitiligo (white spots on the skin) seems to indicate a positive response of the drug. Good news to me at least :)
Caroline Roberts put it very cautiously that "at least it is not very bad [for the patients]" at EADO 2015.
Even though the dataset was small (N = 67), out of the 17 people who got vitiligo only two of them had a progressive decease. But the rest were either stable (3), or had partial (9) or complete (3) regression. So if you get vitiligo it might mean good news, but a larger dataset is needed to show if it is statistically significant.