THE United Nations has set July 31 2000 as the date for a referendum on the future of Western Sahara, according to a UN report released on Tuesday. The referendum to decide the future of the former Spanish colony annexed by Morocco in 1975 will cap a year of identification and appeal processes approved by UN Secretary General Kofi Annan. Voters will choose between the Moroccan flag, or independence, as demanded by the Polisario Front, an opposition movement backed by neighboring Algeria.