Image Quality Measures Calculation Methods

I need to calculate different IQMs like MSE, PSNR, AD, NCC etc for RGB images. My query is:

Can I calculate these after converting to grayscale? i.e. using rgb2gray? Or it is must to input the RGB image?

For Exmple: I am calculating MSE using,

mse = immse(origImg,distImg) %Method 1 (These images are RGB)

and

origImg = rgb2gray(origImg); % Method 2

distImg = rgb2gray(distImg);

origImg = double(origImg);

distImg = double(distImg);

[M N] = size(origImg);

error = origImg - distImg;

MSE = sum(sum(error .* error)) / (M * N);

are giving different results. Why??