rainbowText.R

rainbowText <- function(text, colorFun = grDevices::rainbow, ...) {
  text <- trimws(text)
  # colorspace has some good fucntions that you can use here
  colors <- colorFun(nchar(text), ...)
  # wrap each letter in a differently colored span
  html <- paste0(
    Map(function(letter, color)
      paste0("<span style='color:", color, "'>", letter, "</span>"),
      strsplit(text, "")[[1]], colors
    ), collapse = ""
  )
  
  # create a tempfile and view in RStudio
  tf <- tempfile(fileext = ".html")
  writeLines(html, tf)
  rstudioapi::viewer(tf)
}

rainbowText("hello world")