"Custer himself has been depicted arrayed in a short jacket, an enormous red tie, and long red hair falling over his shoulders."https://www.kshs.org/p/the-pictorial-record-of-the-old-west-4/13042
"He had freckles, and reddish-blonde hair."https://www.esquire.com/news-politics/news/a23270/introducing-george-armstrong-custer-062813/
"Deep blue eyes and a shock of reddish blonde hair rounded out his features."https://truewestmagazine.com/the-civil-war-custer/
"He was an over the top character who loved publicity and gained more than other more accomplished officers, the press for their part loved him a young showman with long red hair and a taste for velvet jackets with gold braid he would not have been out of place in Napoleon's cavalry of half a century earlier."http://www.historyofwar.org/articles/people_custer.html
However, Plains Indians used to calling him "Yellow hair", so it's possible either he was a strawberry blond or, over the years, his hair turned to a sandy shade, which is not unusual for redheads before their hair becomes white.