In my monochrome dabblings, I’ve been getting decent results with a gray tool that allows adjustment of the contribution of the three channels to the gray tone. rawproc’s gray tool looks like this:
Those numbers are used to multiply each corresponding R, G, and B value to come up with the perceptually pleasing gray tone that pixel represents.
In my film shooting, I used to use red filters to turn blue skies black and make cumulus clouds punch out of the image. If you think of what a red filter is doing, the corresponding settings in the gray tool make sense:
to make images look like this:
Red is easy; for other filter colors, look up their 0-255 RGB values in Google and divide each number by 256.