1) Go into your inorganic stone_mineral and stone_layer raw files, search for flux. Change the[REACTION_CLASS:FLUX] to [REACTION_CLASS:SHMUX] or whatever, just some other word.
2) Go into your reactions_smelter file and search for flux again. Make the same change, so that the steel production reactions now require SHMUX.
3) Go back to inorganic stone_layer, and add [REACTION_CLASS:FLUX] to the end of every single sedimentary stone (including the SHMUX stones, which should have both tags).
Now when you go to embark, the FLUX white text indicator on the map will no longer actually indicate flux, but will instead indicate where sedimentary layers exist.
And since virtually all sedimentary layer sites have flux anyway, it pretty much also means flux at the same time.
Really helps in increasing your efficiency at finding sites you want. (Note that sedimentary and igneous extrusive are mutuall exclusive, too, so it also means "not igneous extrusive")
Theoretically you could also go add all your own custom metal smelting reactions equal to default, and then add [ORE:USELESS_METAL*] or whatever to any other mineral you want an indication for, and then use "shallow metals" and "Deep metals" to tell you other things, that you control the meaning of. For example, set it so that only sand layers and iron ore count as "metals" this way, then "shallow metals" plural pretty much always means "sand and iron" for example. Or just add it to all metals except ones you don't care about, so that "deep metals" means "copper and iron, for sure" and never "zinc and platinum". Or whatever else! You could make it indicate diamonds (deep) and magnetite (shallow) and make only one kind of sand (other shallow) and candy(other deep) and be able to verify 100% for sure that all of those exist on a map if it says "shallow metalS" and "deep metalS." You imagination's the limit.
This is much more involved and complicated, though. I might make a mod for it if anybody cares or has a good idea what to use that as an indicator for.
*(USELESS_METAL being just some random non-fire safe metal that you can't build anything with, has a value of 1. Or it could have a boiling point below room temp so it just disappears if you ever smelt it using the auto added reaction).