However, there could be a valid explanation for their sartorial choices - neither Trump nor Prince William are Catholic. The Roman Catholic Church has deep-rooted traditions, including strict guidelines for funeral attire.
Black is the standard color for a Catholic funeral, symbolizing mourning and offering respect to the departed. Other faiths do not mandate black attire as strictly, although it is commonly accepted as appropriate funeral wear.
Trump, a nondenominational Christian, and Wills, from the Church of England, may have decided not to don black attire as a sign of their non-Catholic faith.
https://www.irishstar.com/news/boston-news/real-reason-prince-william-donald-35124019
I am sure William did not wear blue by accident or not knowing what is normal.