Panchromatic images supply high spatial resolution information but not spectrally, while

multi-spectral images supply rich spectral information that can identify but not spatially. 

yes it effects gray information of the mr but preserves the color information , so for denoising of pan image before going to fusion.

for basic concept refer the link

C. Pohl, J.L. Genderen.: “Multi-sensor Image Fusion in Remote

Sensing: Concepts, Methods and Applications”. International Journal

of Remote Sensing, Vol. 19 (5) (1998), pp. 823-854