palette_okabe_ito() is a wrapper around palette.colors() that returns the
Okabe-Ito palette in R 4.0.0 or greater. palette_okabe_ito() returns an
unnamed vector of colors for better use with ggplot2. Additionally, black is
the last color returned by palette_okabe_ito() when 9 colors are needed
instead of the first, as in palette.colors().