The following is an example:
% Sample data
x = linspace(0,4*pi,1000);
y = sin(x);
% Extract positive and negative part
yp = (y + abs(y))/2;
yn = (y - abs(y))/2;
% Plot the data using area function
figure
area(x,yp,'FaceColor','r')
hold on
area(x,yn,'FaceColor','b')
