Decreases the value of the Munsell colour by 1.
darker(col, steps = 1)character vector of Munsell colours
darker("5PB 3/4")
#> [1] "5PB 2/4"
cols <- c("5PB 3/4", "5Y 7/8")
p <- plot_mnsl(c(cols, darker(cols), darker(cols, 2)))
p + ggplot2::facet_wrap(~ names, ncol = 2)