Try this:
rgbImage = imread('peppers.png'); % Read in example image shipped with MATLAB
labImage = rgb2lab(rgbImage);
[LImage, AImage, BImage] = imsplit(labImage);
Then call histogram on each image.
figure;
subplot(4, 2, 1);
imshow(rgbImage);
title('Original RGB image')
subplot(4, 2, 3);
imshow(LImage, []);
title('L Image')
subplot(4, 2, 4);
histogram(LImage);
title('Histogram of L image');
subplot(4, 2, 5);
imshow(AImage, []);
title('A Image')
subplot(4, 2, 6);
histogram(AImage);
title('Histogram of A image');
subplot(4, 2, 7);
imshow(BImage, []);
title('B Image')
subplot(4, 2, 8);
histogram(BImage);
title('Histogram of B image');
Or see my attached demo and adapt it to LAB color space.
