The default colororder has 7 colors in it, and each series uses the next color in the colormap. Once the 7th line is used, the colororder starts over at color 1.
z = 1:100;
x(:,1) = z/10;
x(:,2) = sin(z);
x(:,3) = z/5;
x(:,4) = cos(z);
y = -x;
plot(z,x,z,y)
legend
If you do not want to specify the color, then the next option is to set the colororder using only 4 colors.
newcolors = [0.83 0.14 0.14
1.00 0.54 0.00
0.47 0.25 0.80
0.25 0.80 0.54];
figure
colororder(newcolors)
plot(z,x,z,y)
legend
