Most plotting functions have a col= field, allowing you to
select a color or colors for a plot. This field takes a numeric argument
and selects the matching color from the active color scheme. For lines in
the ``white on black'' (default) color scheme, 0 is black and 1 is white.
You can edit the color scheme (via the menu in the graphics window) and
add some more colors. For instance, if you wrote ``red''
and ``yellow'' after ``white'' on the list of colors for lines,
abline(a,b,col=2) would produce a red line and abline(a,b,col=3)
would produce a yellow line.