How can I change the line color in a graph using plot?

to plot this figure

clc;
clear all;
close all;

fs = 1000;
t = 0:1/fs:0.2;

Vm = 720; % Volt
f0 = 50 ;% Hz
q = 0.5 ; % Voltage Gain
alpha = 0;

Va = Vm*q*cos(2*pi*f0*t + alpha);

Vb = Vm*(q + 0.2*q*(t == 0.1)).*cos(2*pi*f0*t + alpha);

figure,
plot(t,Va,'r',t,Vb,'b');
xlabel('Time');
ylabel('Va and Vb');

[Walter Roberson]

Your code

 plot(t,Va,'r',t,Vb,'b');

already sets the color of the Va plot to be red and of the Vb plot to be blue.