monthly boxplot of two stations in one graph

tidyverse
2

Use the fill aesthetic, see this example:

library(ggplot2)

zoo <- data.frame(stringsAsFactors=FALSE,
    Rainfall = c(15, 53.7, 12.9, 4.1, 76.9, 0, 0, 15.9, 0, 0, 
                 12, 54.3, 11, 3.2, 67.4, 1, 2, 14, 0, 1),
       Month = c("Jan", "Jan", "Jan", "Jan", "Jan", "Feb", "Feb", "Feb", "Feb", "Feb",
                 "Jan", "Jan", "Jan", "Jan", "Jan", "Feb", "Feb", "Feb" , "Feb", "Feb"),
     Station = c("Island", "Island", "Island", "Island", "Island", "Island",
                 "Island", "Island", "Island", "Island", "Inland", "Inland", "Inland",
                 "Inland", "Inland", "Inland", "Inland", "Inland", "Inland", "Inland"),
        Year = c(1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 
                 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993)
)

ggplot(zoo, aes(x = Month, y = Rainfall, fill = Station)) +
    geom_boxplot()

If you need more specific help, please provide a proper REPRoducible EXample (reprex) illustrating your issue.