How to quantify the image quality from the given image samples

Dear MATLAB users,

I have three image samples derived from an original RGB image.

1) Image 1 = R / (R+G+B) <http://i335.photobucket.com/albums/m466/blakeex/Picture2.png>

2) Image 2 = G / (R+G+B) <http://i335.photobucket.com/albums/m466/blakeex/Picture3.png>

3) Image 3 = B / (R+G+B) <http://i335.photobucket.com/albums/m466/blakeex/Picture4.png>

How can I quantify the image quality from three images? What are the formulas or built-in MATLAB functions to calculate the SNR and contrast of those images?

Thank you.