I don't know what shade() you're using, since it's not part of MATLAB. Otherwise, you can do something with patch objects:
y = linspace(0,1,100);
x1 = (1+0.1*randn(1,100)).*(1-y).^0.25;
x2 = 2*(1+0.1*randn(1,100)).*(1-y).^0.25;
hold on
patch([x2 fliplr(-x2)],[y fliplr(y)],'r','facealpha',0.3,'edgecolor','r');
patch([x1 fliplr(-x1)],[y fliplr(y)],'b','facealpha',0.3,'edgecolor','b');
xlim([-2.5 2.5])