Try this:
rgbImage = imread('60_N1_4.png');
subplot(4, 1, 1);
imshow(rgbImage);
axis('on', 'image')
croppedImage = rgbImage(1:121, 1:641, :);
subplot(4, 1, 2);
imshow(croppedImage);
axis('on', 'image')
grayImage = rgb2gray(croppedImage);
lowThreshold = 0;
highThreshold = 150;
% Interactively and visually set a threshold on a gray scale image.
% https://www.mathworks.com/matlabcentral/fileexchange/29372-thresholding-an-image?s_tid=srchtitle
% [lowThreshold, highThreshold] = threshold(0, 120, grayImage)
binaryImage = grayImage >= lowThreshold & grayImage <= highThreshold;
% Get rid of blobs touching the edge of the image.
binaryImage = imclearborder(binaryImage);
binaryImage = imdilate(binaryImage, true(5));
subplot(4, 1, 3);
imshow(binaryImage);
% Fill in that region.
[r, g, b] = imsplit(croppedImage);
r = regionfill(r, binaryImage);
g = regionfill(g, binaryImage);
b = regionfill(b, binaryImage);
rgbImage2 = cat(3, r, g, b);
subplot(4, 1, 4);
imshow(rgbImage2);
