Transparent equivalent of given color

I think is more common to specify alpha in [0,1]. This function do that, plus accept several colors as arguments:

makeTransparent = function(..., alpha=0.5) {

  if(alpha<0 | alpha>1) stop("alpha must be between 0 and 1")

  alpha = floor(255*alpha)  
  newColor = col2rgb(col=unlist(list(...)), alpha=FALSE)

  .makeTransparent = function(col, alpha) {
    rgb(red=col[1], green=col[2], blue=col[3], alpha=alpha, maxColorValue=255)
  }

  newColor = apply(newColor, 2, .makeTransparent, alpha=alpha)

  return(newColor)

}

And, to test:

makeTransparent(2, 4)
[1] "#FF00007F" "#0000FF7F"
makeTransparent("red", "blue")
[1] "#FF00007F" "#0000FF7F"
makeTransparent(rgb(1,0,0), rgb(0,0,1))
[1] "#FF00007F" "#0000FF7F"

makeTransparent("red", "blue", alpha=0.8)
[1] "#FF0000CC" "#0000FFCC"

Converting valuable comment to answer:

Use alpha from package scales - first argument is colour, second alpha (in 0-1 range).

Or write function overt it:

makeTransparent <- function(someColor, alpha=100) scales::alpha(someColor, alpha/100)

Have you had a look at ?rgb?

Usage:

rgb(red, green, blue, alpha, names = NULL, maxColorValue = 1)

An alpha transparency value can also be specified (as an opacity, so ‘0’ means fully transparent and ‘max’ means opaque). If alpha’ is not specified, an opaque colour is generated.

The alpha parameter is for specifying transparency. col2rgb splits R colors specified in other ways into RGB so you can feed them to rgb.

There is a function adjustcolor in grDevices package coming with the base installation of R, that works like this in your case:

    adjustcolor( "red", alpha.f = 0.2)