The tiles I got from maps-for-free.com are a bit too bright for my purposes, so I built a workflow to adjust them.
1) adjust an example image (like this one.) to your liking (I used the GIMP to produce this.)
2) make sure you're using v6.5.3-4 or higher of ImageMagick (first version that supports hald-clut)
3) create a hald colour lookup table with IM like this: "convert hald:8 adjsuted_image.png hald-clut.png". The output image is a palette of the colours used in your adjusted image.
4) test things out by doing this: "convert original_file.png hald-clut.png -hald-clut adjusted_version.png"
5) in my case the results weren't quite right, so I ran my GIMP preset against the hald-clut.png to adjust the palette and then ran #4 again.
6) write a script that recursively runs this command on all of your images, like this: "for i in $( find ./ -name *.jpg); do convert $i hald-clut.png -hald-clut $i; done;"

The results can be seen right now at zoom level 6 across the Mediterranean here: http://hcmc.uvic.ca/grs/map/index2.html