You can desaturate the textures inside the material room if you want to. In the advanced view of the material room, right-click and select New node->Math->hsv. Connect it like in the attached picture. (You would have to do this for each material of M4 to greyscale him completely)

 

In the picture I have set the saturation value to 0 which has made the image texture for the torso greyscale, as seen in the quick render next to the materials.