There is no mention of flagstaff heads, finials, toppers, or ornaments in the US Flag Code (Title 4); this is an observation verified by the American Legion in the flag Q&A section.
However, Army Regulation 840-10, Chapter 8: “Flagstaffs and Flagstaff Heads (Finials)” offers some guidance. In section 8-2 it designates the eagle finial is reserved for the Presidential flagstaff and the spearhead is the only device used with all other Army flagstaffs. A ball is the designated decorative ornament at the top of an installation flagpole.
Nonetheless, the spread eagle finial is widely used by the Army atop the national color for a number of Presidential and DOD officals and the Navy has a number of practices for the use of a range of flagstaff ornaments.
Lastly, the spread eagle finial is widely used as the indoor flagstaff ornament for the national colors for civilian and civic displays. For example, the Veterans of Foreign Wars (VFW) uses the spread eagle finial on top of the national colors at the National Convention, the Army Spear (spade) on service flags, and the Spear (Roman, universal, round, or staff) on organizational flags.
In effect, the spread eagle finial above the national colors is widely used indoor and outdoors, but if you want to emulate a military color guard, stick with the spear.
TC 3-21.5: DRILL AND CEREMONIES. Army Publishing Directorate, Army Publishing Directorate, 03 May 2021.
AR 840-10: FLAGS, GUIDONS, STREAMERS, TABARDS, AND AUTOMOBILE AND AIRCRAFT PLATES. Army Publishing Directorate, Army Publishing Directorate, 17 July 2020, https://armypubs.army.mil/ProductMaps/PubForm/Details.aspx?PUB_ID=1020435.
“Are There Any Specifications Regarding the Flag Pole or Flag Pole Toppings?” The American Legion, The American Legion, https://www.legion.org/flag/questions-answers/91474/are-there-any-specifications-regarding-flag-pole-or-flag-pole-toppings.
“Title 4—Flag And Seal, Seat Of Government, And The States.” 4 USC Ch. 1: The Flag, Office of the Law Revision Counsel of the United States House of Representatives, https://uscode.house.gov/view.xhtml?path=%2Fprelim%40title4%2Fchapter1&edition=prelim.