library("ggplot2")

ggplot(data = warpbreaks, aes(x = wool, y = breaks)) +
  geom_boxplot(
    notch = T,
    fill = c("blue", "red", "green3", "hotpink2"),
    outlier.color = NULL
  ) +
  theme(legend.position = "none") +
  theme_classic()
#> Error: Aesthetics must be either length 1 or the same as the data (2): fill