How to convert image from clustering to Gray image?

i work in blood cell image , i used clustering to extract red blood cell only without white blood cell i did it and its work in this code , my problem is i want to convert the final image to gray image or RGB , i don't know the type of the final image(blue_nuclei) the image i want to convert to gray named is (blue_nuclei) in code this is my code :

he = imread('CL_13-09_image2.jpg');

figure(1),imshow(he), title('H&E image');

cform = makecform('srgb2lab');

lab_he = applycform(he,cform);

ab = double(lab_he(:,:,2:3));

nrows = size(ab,1);

ncols = size(ab,2);

ab = reshape(ab,nrows*ncols,2);

nColors = 3;

% repeat the clustering 3 times to avoid local minima

[cluster_idx cluster_center] = kmeans(ab,nColors,'distance','sqEuclidean', ...

                                        'Replicates',3);

pixel_labels = reshape(cluster_idx,nrows,ncols);

figure(2),imshow(pixel_labels,[]), title('image labeled by cluster index');

segmented_images = cell(1,3);

rgb_label = repmat(pixel_labels,[1 1 3]);

for k = 1:nColors

   color = he;

      color(rgb_label ~= k) = 0;

      segmented_images{k} = color;

end

figure(3),imshow(segmented_images{1}), title('objects in cluster 1');

   figure(4),imshow(segmented_images{2}), title('objects in cluster 2');

figure(5),imshow(segmented_images{3}), title('objects in cluster 3');

mean_cluster_val = zeros(3,1);

for k = 1:nColors

      mean_cluster_val(k) = mean(cluster_center(k));

end

[mean_cluster_val,idx] = sort(mean_cluster_val);

blue_cluster_num = idx(2);

L = lab_he(:,:,1);

blue_idx = find(pixel_labels == blue_cluster_num);

L_blue = L(blue_idx);

is_light_blue = im2bw(L_blue,graythresh(L_blue));

nuclei_labels = repmat(uint8(0),[nrows ncols]);

nuclei_labels(blue_idx(is_light_blue==false)) = 1;

nuclei_labels = repmat(nuclei_labels,[1 1 3]);

blue_nuclei = he;

blue_nuclei(nuclei_labels ~= 1) = 0;

figure(6),imshow(blue_nuclei), title('blue nuclei');

my image i work on :