Have a loop. For each boundary that you have, call poly2mask. Then "OR" it in to a binary image that you will be building up. Then cast your image to color if necessary, then set masked areas to green. I'll give you pseudocode. See if you can figure it out.
binaryImage = false(rows, columns);
for k = 1 : length(boundaries)
thisBoundary = boundaries{k};
x = thisBoundary(:,1);
y = thisBoundary(:, 2);
thisMask = poly2mask(x, y, rows, columns);
binaryImage = binaryImage | thisMask;
end
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
redChannel(binaryImage) = 0;
greenChannel(binaryImage) = 255;
blueChannel(binaryImage) = 0;
% Recombine separate color channels into a single, true color RGB image.
rgbImage = cat(3, redChannel, greenChannel, blueChannel);
imshow(rgbImage);
