@mherbert it's a single-layer, actually! Which means the "moat" glows beautifully when backlit.
The palette is:
Yellow (unused) - No copper or solder mask
Dark green (parks) - Solder mask over no copper
Lighter Green - Solder mask over copper
Gold (roads) - ENIG finish
White (water) - silkscreen
This design also has a full green solder mask on the back. I'm not sure whether additional layers could change the appearance from the front much, you could probably affect the backlit appearance!