Named vector with the 657 named colors in the standard X11 color
specification, after removing spaces and normalizing to lowercase
names. These are the default named colors in R.
EXAMPLES:
# Use the X11 color definitions to resolve names such as "gray87"
add.color.values(x11.colors)
# Use the X11 color definitions and overwrite existing definitions
# if conflicts occur
add.color.values(x11.colors, overwrite=T)