The basic yellow filter for monochrome work cuts out blue light. What you could do is split the image into an RGB set, darken the blue image, and recombine. Then convert to greyscale. For an unfiltered B+W image I reckon you should brighten the blue image, since B+W films are more sensitive to blue light. The other filters used will chop out more and more. One of the problems with digital is that you throw away a lot of spectral information. You keep the detail our eyes can see, but throw away some of what filters work on. Oh, and you know that green plants reflect a lot of red light?