It is currently not possible to place down a marker with grey color directly in editor interface because this color is not available in the list.
It would be usefull to allow it.
Example : I place down a new building in editor and I want the players to be able to see it on the map as every other buildings on the map. This is currently not possible unless using scripting commands as ' "M_custombuilding_01" setMarkerColor "ColorGrey";