image_col_numeric: Colorize an Image according to Gray Scale
In plothelper: New Plots Based on 'ggplot2' and Functions to Create Regular Shapes

A color image can be converted to one with different degrees of gray. Then, colors in a palette can be added according to the gray degrees. The function is a simple wrapper of scales::col_numeric. The pixels which are deliberately assigned "transparent" in the original magick image will always kept unchanged.

image_col_numeric(
  x,
  palette = c("purple", "yellow"),
  n = 256,
  alpha = FALSE,
  result = "magick",
  res = 144
)

`x`	an image read into R by `magick::image_read`.
`palette`	two or more colors. The default is c("purple", "yellow") which means the deeper colors on the image will become purple and the lighter yellow.
`n`	the max num of colors that will be used. The default is 256. Note, the number of colors that really exist may be smaller than this number.
`alpha`	whether transparency is used. Transparency only exists when `alpha = TRUE` and your image is in the format (e. g., png) that supports transparency. The default is FALSE.
`result`	if it is "magick" (default), the result is a picture of the same type used by package magick. If it is "raster", the result is a matrix that can be used as a raster by `ggplot2::annotation_raster`.
`res`	resolution that is used by `magick::image_graph`. The default is 144.