SQL> declare
  2    TYPE COLOR_AT IS TABLE OF VARCHAR2(10); --INDEX BY BINARY_INTEGER
  3    v_set_color_a COLOR_AT := COLOR_AT('Red','Green','Blue');
  4    v_set_color_b COLOR_AT := COLOR_AT('Yellow');
  5  begin
  6    v_set_color_a := v_set_color_a MULTISET UNION v_set_color_b;
  7    for i in v_set_color_a.first..v_set_color_a.last loop
  8      dbms_output.put_line (v_set_color_a(i));
  9    end loop;
 10  end;
 11  /
Red
Green
Blue
Yellow

PL/SQL procedure successfully completed.

v_set_color_a := v_set_color_a MULTISET UNION v_set_color_b;