Export >> Binary >> Export Colour Map
Export >> Text >> Export Colour Map
Direct editing of the map cell colours is not possible because the program does not support "per map cell" colouring, only per map, per tile or per char, the above (export) options are included as a work-around.
It is just a great deal more optimal (when limited to 64KB) to associate colours with maps, tiles or chars, hence the option you require does not exist.
Sorry.
Your support is greatly appreciated, feel free to convince me to add such an option but weaving it in to the existing code would be months of work, would further complicate an already complex system and I have yet to see it used in any commercial game.