There are three possible outcomes when the command is selected based on the current selection as follows.
1/ If no entities are selected then all of the entities with the current colour are selected.
2/ If a single entity is selected then all entities with the same colour are selected.
3/ If multiple entities are selected then any item using any of the colours of those entities are then selected.
This command is particularly useful to select all items with a specific colour(s) to either:-
1/ Delete them.
2/ Move or copy them etc.
3/ Change the properties either colour, layer, style etc.
4/ As a visual check to see if entities are or not using the correct colour.