Changing the default color scheme in ggplot2

UPDATE: see update below based on comments from nmarkgraf. UPDATE 2: I changed the theme to theme_minimal thanks to the comment from @neuwirthe. UPDATE 3: A more efficient way to plot a discrete scale using viridis. Thanks to flying sheep; see way 4 below The default color scheme in ggplot2 is suitable for many purposes, but, for instance, it is not suitable for b/w printing, and maybe not suitable for persons with limited color …