您需要在aes中映射颜色:

ggplot(aes(x = Sepal.Length), data = iris) + 
  geom_histogram(color = 'black', fill = NA) + 
  geom_vline(aes(xintercept=median(iris$Sepal.Length),
                 color="median"), linetype="dashed",
             size=1) +
  geom_vline(aes(xintercept=mean(iris$Sepal.Length),
                 color="mean"), linetype="dashed",
             size=1) +
  scale_color_manual(name = "statistics", values = c(median = "blue", mean = "red"))