Segmentation might be a bit easier with a bit of a diffuser to deal with the bg. The median filter (or just downscaling the image) helps deal with it in this case.
A = imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/905805/Main.jpg');
A = rgb2gray(A);
A = medfilt2(A,[10 10]);
mask = imclearborder(A<150);
mask = bwareaopen(mask,2000);
imshow(mask)
S = regionprops(mask,A,'meanintensity','centroid');
C = vertcat(S.Centroid);
graincolor = vertcat(S.MeanIntensity);
grainmask = graincolor<60;
figure
imshow(A); hold on
for b = 1:numel(S)
if grainmask(b)
plot(C(b,1),C(b,2),'bo','markersize',25);
else
plot(C(b,1),C(b,2),'ro','markersize',25);
end
end
