why false function comes up with duotone image?

Hi I have a dicom image from ultrasound. I applied a manual segmentation mask using imageSegmenter. I pressed generate function at the Export option.

There is a point that creates an empty mask using the false function

% Create empty mask. BW = false(size(X,1),size(X,2));

From that point and on when I try to imshow the image, it comes with a black and white version of it, where black is the tissue and white the segmented mask.

I don't want this. I want to keep the white area mask (maybe lower its opacity a bit) and also keep the gray tissue of an ultrasound image. Not a black one. How do I do that?

Thanks in advance