a color object or vector of any of the three kinds of R color
specifications, i.e., either a color name (as listed by colors()), a
hexadecimal string of the form "#rrggbb" or "#rrggbbaa" (see rgb), or a
positive integer i meaning palette()[i].
alpha
Numeric between 0 and 1. 0 will result in full transparency and
1 results in no transparency.