Probably because it's color. Use histogram() or else use imhist() on each color channel one at a time.
% Extract the individual red, green, and blue color channels.
redChannel = im(:, :, 1);
greenChannel = im(:, :, 2);
blueChannel = im(:, :, 3);
[countR, grayLevelsR] = imhist(redChannel);
[countG, grayLevelsG] = imhist(greenChannel);
[countB, grayLevelsB] = imhist(blueChannel);
Or convert to gray scale:
grayImage = rgb2gray(im);
[countR, grayLevelsR] = imhist(grayImage);
