There have been a few times in the past (such as when I was preparing the base mapping for My Wycombe) when I've wanted to create greyscale base mapping using MapServer. Previously I've experimented with various processing options for raster images (with varying degrees of success), converting the source images using ImageMagick and converting colour RGB values to their greyscale equivalent for vector features and it's generally been time consuming. Today the requirement came up again and I discovered a far simpler option: specifying a greyscale palette as an OUTPUTFORMAT FORMATOPTION. The palette simply contains grey values from 0,0,0 (black) to 255,255,255 (white) and all values in between. MapServer then does the job of converting the colours for your source data into their greyscale equivalents.

In the map file specify an OUTPUTFORMAT that used a specific palette:

# Specify a plain PNG output with a
# greyscale palette
IMAGETYPE "AGG_PNG"
OUTPUTFORMAT
    NAME  "AGG_PNG"
    DRIVER  "AGG/PNG"
    MIMETYPE "image/png"
    IMAGEMODE "RGB"
    EXTENSION "png"
    FORMATOPTION "INTERLACE=OFF"
    FORMATOPTION "PALETTE_FORCE=ON"
    FORMATOPTION "PALETTE=grey.txt"
END

Snippet from the palette file (download the full version):

0,0,0
1,1,1
2,2,2
--- you get the idea ---
253,253,253
254,254,254
255,255,255

BTW I used the following Python to generate the values for the palette:

for i in range(0,256):
    print '%i,%i,%i' % (i,i,i)