short name for the colormap. It can be one of "jet"
(jet colormap), "bwr" (blue-white-red colormap), "gbr" (green-black-red
colormap), "wyr" (white-yellow-red colormap), "br" (black-red
colormap), "yr" (yellow-red colormap), "wb" (white-black colormap),
"rainbow" (rainbow colormap, that is,
red-yellow-green-cyan-blue-magenta), and "ggplot2" (emulating ggplot2
default color palette). Alternatively, any hyphen-separated HTML color
names, e.g. "lightyellow-orange" (by default), "blue-black-yellow",
"royalblue-white-sandybrown", "darkgreen-white-darkviolet". A list of
standard color names can be found in
http://html-color-codes.info/color-names