Try this:
Time = 0:200;
Wetbulb = 0.005*(Time-100).^2 + rand(size(Time))*2 + 50;
green=20+Time*0;
yellow=28+Time*0;
red=100+Time*0;
base=0+Time*0;
greenarea=[base,fliplr(green)];
basearea=[Time,fliplr(Time)];
figure(5)
plot(Time,Wetbulb);
title('Wetbulb Temperature vs. Time');
xlabel('Time');
ylabel('Wetbulb Temperature');
axis([0 200 0 100]);
hold on
plot(Time,green)
plot(Time,yellow)
plot(Time,red)
fill(basearea,greenarea,'g', 'FaceAlpha',0.2)
fill(basearea,[green fliplr(yellow)],'y', 'FaceAlpha',0.2)
fill(basearea,[yellow fliplr(red)],'r', 'FaceAlpha',0.2)
hold off
To get patch and fill to work as you want them to, you need to create a closed area, then fill it. I did this here by taking the x-vector for each segment and horizontally concatenating it with a flipped version of itself. (That is the same for all segments.) The y-vector for the green area was a vector of zeros horizontally concatenated with the flipped upper limit. The y-vector for the yellow area was the upper border of the green area horizontally concatenated with the flipped upper border of the yellow area. The same idea applies to the red area. I know your lines are constant and horizontal, so flipping them here does not really matter, however that may not always be the situation (for example with varying borders), so I prefer to use that convention for all patch and fill calls in my code.
I do not have your data, so I created a ‘Wetbulb’ vector. This code should work with your data with little or no alteration, if I understand correctly what you want. I also produced ‘shading’, not solid colours, since you mentioned that. If you want solid colours, you will have to put this line last:
plot(Time,Wetbulb)
so it overplots the red fill region. Otherwise the red fill will hide it.
Experiment to get the result you want.
