Digging in the files i can't find the reason why Factorissimo has a tile of each type for each factory building. I get the need for colours for the side and pattern, but that aside it's 4 valuable tile slots taken for no obvious reason. Is it a remnant of the past that can't be taken out for save compatibility or is there a technical reason for their continued existence ?

There is in fact a difference between these tile types, namely a slight change in tint on the minimap. It should be perfectly possible to unify the tile types with an appropriate Factorissimo2 update and migration though. I've thought about merging these tiles, but I didn't find it a priority as I didn't really expect Factorio to keep the narrow tile limit for such a long time.

Alien biomes makes modlists short on tiles, Yuoki isn't helpful as there's no settings, and after review, i took note that factorissimo is the second highest tile user in my set when you take https://mods.factorio.com/mod/space-factorissimo-updated into account. Crushing all the grey tiles together would be nice (+ 8 slots on both mods) but'd require some form of cooperation. If space factorissimo crushes their cyan tiles together that's an extra 2 tiles saved.