24
I am not sure if color really helps in this graph, since it is already clear what each boxplot corresponds to. However, if you still need to color this in black and white, you can achieve it using scale_fill_grey. Here is an example
library(ggplot2)
data(tips)
p0 = qplot(day, tip/total_bill, data = tips, geom = 'boxplot', fill = day) +
scale_fill_grey()
print(p0)
This produces the output shown below
