ax.FontSize = 20;
ax is undefined in the above code...there may have been another axes in workspace, but it isn't going to be addressing the one here.
figure(1)
hold on
grid on
grid minor
contour(real(z),imag(z),imag(f),[-5:.2:5])
plot(real(z_circle),imag(z_circle),'b')
hAx=gca; % get current axes
axis equal
axis([-5 5 -5 5])
%hold on % hold is already on...superfluous
contour(real(J),imag(J),imag(f),[-5:.2:5])
fill(real(z_airfoil),imag(z_airfoil),'w')
%axis equal % dito
%axis([-5 5 -5 5])
hAx.FontSize = 20; % use current handle
xlabel('Real (x)');
ylabel('Imaginary (y)');
box on
set(hAx,'GridLineStyle','-','MinorGridLineStyle','-')
set(hAx,'GridColor','k','MinorGridColor','k')
You forgot to attach the needed data so anybody could possibly try to duplicate any issues about the color...
