I'm finding that the garish colour scheme of the maps-for-free tiles are a bit much for this application. Points, in any colour combination tend to get lost in the intense range. So, I've been fiddling with some setting in the GIMP, and I have a more subdued scheme to try out.
I need a script (imagemagick?) that will adjust the tiles thusly:
In the GIMP > Colours > Hue-Saturation dialog, I made the following adjustments:
Master: sat=-25, hue=15
R: sat=-45, light=20, hue=1-
Y: sat=-35, light=45
G: sat=-50(-80 light=18)
C: sat=-45 (overlap=40)
B: sat=25, light=-10
and I think it looks better at least.

Now, I need to script something that will troll through all 17 million images and adjust each one with these settings...