Colour is an aesthetic (plot parameter) that changes the bar outline color.
See also:
A name, e.g., “red”. R has 657 built-in named colours, which can be listed with colours(). The Stowers Institute provides a nice printable pdf that lists all colours.
An rgb specification, with a string of the form “#RRGGBB” where each of the pairs RR, GG, BB consists of two hexadecimal digits giving a value in the range 00 to FF
You can optionally make the colour transparent by using the form #RRGGBBAA
An NA, for a completely transparent colour.