Here’s a green one with radius = 0.5 to get you started. You will also need to read the documentation on the hold function.
Example:
t = linspace(0, 2*pi);
r = 0.5;
x = r*cos(t);
y = r*sin(t);
figure(1)
fill(x, y, 'g')
axis([-1 1 -1 1])
axis square
