The fill command can do this. It requires some careful control of the ordering of the points and the ordering of the lines (hence the fliplr in the example below) ...
doc fill
x = 1:10;
y1 = 1:10;
y2 = 2:11;
h = fill([x, fliplr(x)], [y1, fliplr(y2)], 'r')
You can make it transparent with
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