highlightCol: add alpha transparency value to a color
Description
An alpha transparency value can be specified to a color, in order to get better
color for background.
Usage
highlightCol(col, alpha = 0.5)
Arguments
col
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
a value in [0, 1]
Value
a vector of colors in hexadecimal string of the form "#rrggbbaa".