Comparison testing shows that for different maps on the C64 there are different boundaries between the black, blue, green and red levels. Writing down the "boundary" numbers and searching for them shows them on page 1 and after a little more sleuthing (shamusing?) I also have the table where they're stored for different levels. Searching for the same numbers on the Atari reveals nothing but after decreasing them by one I find the list.
While this table is no more than 15 bytes long, I'll still patch the data into the "map data file" to leave the possibility of loading even more maps open.
Object (Keyhole, Key, etc.) colors need some more work as well. Atari Shamus uses a fixed luminance of 6 for all objects but that simply doesn't work when trying to recreate the C64's palette with its five shades of gray. As it would be quite shameful not to be able to recreate the C64 colors on a machine that prides itself on having 16 times as many, I need to store luminance information as well and patch the Atari code to use it. I have an idea how to do that but it remains to be seen whether that will break something else