Darken or lighten colors in R

darken.R

darken <- function(color, factor=1.4){
    col <- col2rgb(color)
    col <- col/factor
    col <- rgb(t(col), maxColorValue=255)
    col
}


lighten <- function(color, factor=1.4){
    col <- col2rgb(color)
    col <- col*factor
    col <- rgb(t(col), maxColorValue=255)
    col
}

# Example:
darkFirebrick <- darken("firebrick")

Here's an example of how to use this with a column vector via sapply():

chromatin_state_colors <- c('Red', 'OrangeRed', 'LimeGreen', 'Green', 'DarkGreen', 'GreenYellow', '#E5E500', '#3D7B66', 'PaleTurquoise', 'IndianRed', 'DarkSalmon', 'DarkKhaki', 'gray50', 'Gainsboro', 'whitesmoke')
lightened_chromatin_state_colors <- sapply(chromatin_state_colors, lighten)
darkened_chromatin_state_colors <- sapply(chromatin_state_colors, darken)

Thanks!
For lighten() the following appeared more natural to me. Larger factors make color more light with a factor of 1 generating white:

lighten <- function(color, factor = 0.5) {
  if ((factor > 1) | (factor < 0)) stop("factor needs to be within [0,1]")
  col <- col2rgb(color)
  col <- col + (255 - col)*factor
  col <- rgb(t(col), maxColorValue=255)
  col
}