Using plot function in Octave 4.4.1 there are some issuses on displaying (multiple) plots.
typing the commands:
plot (x, y, "r", x, z, "b")
or
plot (x, y, "b", x, z, "r")
or
plot (x, y, "b", x, z, "b")
show only the graph(s) with "b" parameter as a RED line.
Other color arguments that "works" are "m" wihch dispay a yellow line and "c" that display a purple line. All the other color arguments are not visible on the plots.