Try this:
subplot(2, 1, 1);
% Display RGB image with yellow boundary burned into it.
imshow(rgbImage); % or display the gray scale image then call hold on and then call plot(xYellow, yYellow, 'y-', 'LineWidth', 3)
subplot(2, 1, 2);
% Mask the image using bsxfun() function to multiply the mask by each channel individually. Works for gray scale as well as RGB Color images.
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, 'like', rgbImage));
imshow(maskedRgbImage);
If you don't have mask, you can get it from the coordinates of the yellow boundary
[rows, columns, numberOfColorChannels] = size(rgbImage);
mask = poly2mask(xYellow, yYellow, rows, columns);
