how can i select a specific color threshold in a image?

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EM geo 2018-10-26

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https://ww2.mathworks.cn/matlabcentral/answers/426223-how-can-i-select-a-specific-color-threshold-in-a-image

i'm trying to select all the black pixels of some white-black tiff images. I have the code below:

fontSize = 20;
imagefiles=dir('*.TIF');
nfiles=length(imagefiles);
a1=imread('a1.TIF');
[rows1 columns2 numberOfColorBands3] = size(a1);
totalNumberOfPixels1 = (rows1 * columns2)*nfiles;
for i = 1:nfiles
     grayImage=imagefiles(i).name;
     currentimage=imread(grayImage);
     figure;
     [rows columns numberOfColorBands] = size(grayImage);
     subplot(2, 2, 1);
     imshow(currentimage)
     title('Original Grayscale Image', 'FontSize', 20);
     [pixelCount grayLevels] = imhist(currentimage);
     subplot(2, 2, 2);
     bar(pixelCount);
     title('Histogram of original image', 'FontSize', fontSize);
     xlim([0 grayLevels(end)]); % Scale x axis manually.
     binaryImage = currentimage > 25;
     subplot(2, 2, 3);
     imshow(binaryImage, []);
     title('Binary Image', 'FontSize', fontSize);
     numberOfBlackPixels = sum(binaryImage == 0);
     numberOfWhitePixels = sum(binaryImage);
     percentBlackPixels = 100.0 * numberOfBlackPixels/totalNumberOfPixels1;
     percentWhitePixels = 100.0 * numberOfWhitePixels/totalNumberOfPixels1;
end
T = table(numberOfBlackPixels,numberOfWhitePixels,percentBlackPixels,percentWhitePixels);
T(1,:)
filename = 'results_neri_187_1.xlsx';
writetable(T,filename,'Sheet',1,'Range','D1')

With this code i can only select the real black pixels. I need to look at the histogram and select all the black and also some dark gray, but i have to assess it each time. How can i select a threshold of colors in matlab and add it in this code?