You want to use different colors for a continuous variable.

In the example here, we’ll specify the colors for a continuous variable using various gradient scales (Figure 12-12). The colors can be named, or they can be specified with RGB values:

library(gcookbook) # For the data set

# Base plot
p <- ggplot(heightweight, aes(x=ageYear, y=heightIn, colour=weightLb)) +
     geom_point(size=3)

p

# With a gradient between two colors
p + scale_colour_gradient(low="black", high="white")

# A gradient with a white midpoint
library(scales)
p + scale_colour_gradient2(low=muted("red"), mid="white", high=muted("blue"),
    midpoint=110)

# A gradient of n colors
p + scale_colour_gradientn(colours = c("darkred", "orange", "yellow", "white"))