基于这里建议的alistaire's answerChange color of specific tick in ggplot2，我们可以做：

library(ggplot2)

    ggplot(iris, aes(x=Species, y=Sepal.Length)) + 
      geom_boxplot() +
      scale_y_continuous(breaks=c(5,6,7,7.5,8), 
                         labels=expression("5", "6", "7", "7.5", "8")) +
      geom_hline(yintercept=7.5, col='dark green', linetype=2) +
      theme(axis.text.y = element_text(colour = c('black', 'black','black', 'dark green', 'black')))

我使用虹膜数据集来演示如何将一个特定的刻度标记标记为绿色。希望它能有所帮助
带有一个绿色的箱形图：

这个方法也适用于 ggpubr：

ggboxplot(iris, x = "Species", y = "Sepal.Length") +
  scale_y_continuous(breaks=c(5,6,7,7.5,8), 
                     labels=expression("5", "6", "7", "7.5", "8")) +
  geom_hline(yintercept=7.5, col='dark green', linetype=2) +
  theme(axis.text.y = element_text(colour = c('black', 'black','black', 'dark green', 'black')))

正如@juan_bzt的回答一样，添加注解的最简单方法是将其添加为（可能是额外的）轴中断。这样做可以避免与轴线重叠，并且不会影响比例范围。
然而，除了将颜色向量传递给element_text的color参数的选项之外，第二个选项是使用ggtext::element_markdown，它也通过一些HTML进行样式化。CSS和/或markdown。在下面的代码中，我将ifelse传递给scale_y_continuous的labels参数，该参数将y=50处的断点 Package 在绿色色的span标记中：

library(ggpubr)
library(ggtext)

ggboxplot(TPdat,
  x = "loc", y = "TP",
  ylab = "Median Annual Total Phosphorus (\U00B5g/L)", xlab = "Sampling Site",
  outlier.shape = NA, ylim = c(0, 925)
) +
  scale_x_discrete(labels = c("1", "2", "3", "4", "5", "6")) +
  theme(
    axis.text = element_text(size = 16, family = "Calibri"),
    axis.title = element_text(size = 16, family = "Calibri")
  ) +
  geom_hline(yintercept = 50, col = "dark green", linetype = 2) +
  coord_cartesian(clip = "off", ylim = c(0, NA)) +
  scale_y_continuous(
    labels = ~ ifelse(
      .x == 50,
      paste0("<span style='color: \"dark green\"'>", .x, "</span>"),
      as.character(.x)
    )
  ) +
  stat_compare_means(
    comparisons = my_comparisons, label = "p.signif",
    symnum.args = symnum.args,
    tip.length = c(0.01, 0.01, 0.01, 0.01, 0.01),
    label.y = c(800, 825, 850, 875, 900)
  ) +
  stat_compare_means(label.y = 930, family = "Calibri", size = 5) +
  theme(axis.text.y = ggtext::element_markdown())