How can i observe a 3 dimensional matrix(hdr) using pcolor command?

It is possible that all of the values are small and so black might be appropriate. You can experiment with.

imagesc(rgb2gray(hdr));
colormap(gray(256))

You cannot get "actual" luminance values from RGB values without photometric calibration. rgb2gray() will give you relative luminance, subject to a number of assumptions (assumptions about linearity of the detector, assumptions about relative intensity of spectral bands, assumptions about the lens being linear out to the edge... The assumption of linearity of the detector is especially problematic at low light levels.)