You could use a colormap such as HSV to generate a set of colors. For example:
cc=hsv(12);
figure;
hold on;
for i=1:12
plot([0 1],[0 i],'color',cc(i,:));
end
MATLAB has 13 different named colormaps ('doc colormap' lists them all).
Another option for plotting lines in different colors is to use the LineStyleOrder property; see Defining the Color of Lines for Plotting in the MATLAB documentation for more information.