It appears that the expected behavior of your show command doesn’t match the definition all your YANG model.
The way your YANG model is defined, ConfD expects for the “all” string to be another instance of your colours list. When “all” is entered as an argument for the show colours command, ConfD will issue an get_elem( ) call for the instance of the colours list with its key that matches “all”.
The auto rendered behavior of the show command for both the J style and C style of CLI will show all entries of your colours list when you enter either of the following:
show colours *
Take a look at Chapter 16.21.6, Adding custom show output, of the ConfD 6.0 User Guide if you would like to customize the behavior of the auto-rendered show commands.