Why images appear that bitmap black and white way?

Dicome pictures are ok graytone when opened on a dicom viewer but appear as bitmap black and white (see attachment). How do I fix it?

N=5; img_dir ='D:\Stelios PhD files\Foot Images'

strfile = 'Z00'; img = dicomread(fullfile(img_dir, strfile)); siz_img = size(img);

% create result matrix: D = NaN([siz_img N]); D(:,:,1) = img;

% load all the remaining images and put them in the matrix for i=1:N strfile = sprintf('Z00',i); D(:,:,i)= dicomread(fullfile(img_dir, strfile)); end

D=squeeze(D);

figure imshow(D(:,:,N)); %transverse plane figure imshow(squeeze(D(:,N,:))'); %sagittal plane figure imshow(squeeze(D(N,:,:))); % coronal plane