How to colour a portion of a curve by red and remaining by black

I want to colour the lower portion of curve by red and broked curve and upper part of the curve by black as shown in the fig attached.

Code

clear all
format long
set(0,'DefaultAxesFontSize',20);
figure
load('EP_EP(1).mat','x'); %load only x
idx = x(3,:)>=0.16 & x(3,:)<=0.305003; 
plot(x(3,:),x(1,:),'k--', 'LineWidth',2); 
xlabel('$a$','FontSize',20,'interpreter','latex','FontWeight','normal','Color','k');
ylabel('$b$','FontSize',20,'interpreter','latex','FontWeight','normal','Color','k');
axis([0 .4 .3 1]);

Answer 1

[Voss]

set(0,'DefaultAxesFontSize',20);
figure
load('EP_EP(1).mat','x'); %load only x

[~,idx] = max(x(3,:));
plot(x(3,idx:end),x(1,idx:end),'k', 'LineWidth',2); % solid black line
hold on
plot(x(3,1:idx),x(1,1:idx),'r--', 'LineWidth',2); % broken red line

xlabel('$a$','FontSize',20,'interpreter','latex','FontWeight','normal','Color','k');
ylabel('$b$','FontSize',20,'interpreter','latex','FontWeight','normal','Color','k');
axis([0 .4 .3 1]);

[Atom]

Thank you, Voss. It's functioning well.

[Voss]

You're welcome! Any questions, let me know.

[Atom]

Just one question: If I want to restrict the lower curve to the range where a=0.25 to 0.305003, and how can this method assist with that?

[Voss]

set(0,'DefaultAxesFontSize',20);
figure
load('EP_EP(1).mat','x'); %load only x

idx_min = find(x(3,:) >= 0.25, 1);
[~,idx_max] = max(x(3,:));

plot(x(3,idx_max:end),x(1,idx_max:end),'k', 'LineWidth',2); % solid black line
hold on
plot(x(3,idx_min:idx_max),x(1,idx_min:idx_max),'r--', 'LineWidth',2); % broken red line

xlabel('$a$','FontSize',20,'interpreter','latex','FontWeight','normal','Color','k');
ylabel('$b$','FontSize',20,'interpreter','latex','FontWeight','normal','Color','k');
axis([0 .4 .3 1]);

[Atom]

Thank you, Voss, for your support. I greatly appreciate your time and effort. My regards.

[Voss]

You're welcome!

Answer 2

[John D'Errico]

Plot it as TWO curves, both on the same figure, one in red, one in black.

Use the hold command between the two calls to plot, so the two curves will be on the same figure.

[Atom]

How to do that modification.... I need some assistance with modifying the code. Thanks a lot.

[Voss]

@Atom: See my answer.