How do I convert convert an infrared image and RGB image to an commun space matlab

How do you convert a RGB image and an IR image to an commun space for purpose to apply a metric of the similarity between this two images? Is it possible using MATLAB?

[Walter Roberson]

Isn't this really just a rewording of http://uk.mathworks.com/matlabcentral/answers/249789-how-to-compare-two-multimodal-images-are-so-similar-spatially-or-not-with-matlab-code ?