how to change the color of lung portion only from binary image?

Answer 1

[Image Analyst]

Invert the lousy image you have (that is if you don't want to improve the segmentation), call imerode(), call imclearborder, call bwareafilt() to take the two largest remaining blobs. Here's a start

se = strel('disk', 0, 50); % Adjust 50 as necessary
mask = imerode(~mask, se);
mask = imclearborder(mask);
mask = bwareafilt(mask, 2);
mask = imdilate(mask, se);
imoverlay(.........

See if you can finish it.