You want to use colors that can be distinguished by colorblind viewers.

Use the palette defined here (cb_palette) with scale_fill_manual() (Figure 12-10):

library(gcookbook) # For the data set

# Base plot
p <- ggplot(uspopage, aes(x=Year, y=Thousands, fill=AgeGroup)) + geom_area()

# The palette with grey:
cb_palette <- c("#999999", "#E69F00", "#56B4E9", "#009E73", "#F0E442", 
                "#0072B2", "#D55E00", "#CC79A7")

# Add it to the plot
p + scale_fill_manual(values=cb_palette)

Figure 12-10. A graph with the colorblind-friendly palette

A chart of the colors is shown in Figure 12-11.

Figure 12-11. Colorblind palette with RGB values

In some cases it may be better to use black instead of grey. To do this, replace the "#999999" with "#000000" or "black":