Yes fair enough, I completely see where you are coming from.
Adding it as a kind of novelty "add on" would be fairly simple compared to trying to add it as an actual proper colouring method.
The latter option is really not trivial as it would need to be worked into every single element of the existing model and all of the existing tools such as the image importer and snapshot ripper etc + almost every operation has to take special consideration when it comes to a project's display mode and colouring method (currently 5x3 possible configurations + tiled/untiled conditions + special care around anything to do with ECM mode).
If it were trivial I obviously would have just added it a long time ago.
So knowing this, you wouldn't care that a project could not be saved/loaded using "per map cell colouring" (a new project file format would need to be designed).
You would be happy to just be able to paint colours directly into the map and save them to a file?
What about the bitmap modes? ie. 2 or 3 colours per cell, how would that work?
What about material attributes? would you want to directly paint those as well?
Just asking, I am genuinely interested to know what you would like.