You could take the original image file, make it darker with some image editing program and add the new texture with a mod and use the new image file. That obviously can’t be done with map.ini alone.
If there is some way to make an image darker in map.ini? I don’t know. Don’t really have a lot of experience with using map.ini.
This is what I usually do when I'm modding M&B Warband, but I'm trying to keep my BFME to map.ini only. The way I see it, making a mod for BFME would be pointless unless I edit enough stuff to make it worth it, like brand new models, skeletons, etc. I kinda like the challenge of only editing excising things from 1.06 and keeping it playable for everyone.