如何将单个scale_color的不同方面映射到ggplot2中的不同变量?

I want to simultaneously visualize the mean and error (variance) of predictions in spatial data. One possible method for doing this would be to map the mean to hue and the error to either saturation or luminance. Although I'm doing this in maps, I'll offer a much simpler minimal working example here adapted from ggplot2's documentation.I want to simultaneously visualize the mean and