Thanks for more information on that. I knew butternuts were like walnuts (both grow in NY, though butternuts are a bit more rare), but I didn’t know they were used for the dyes for Confederate uniforms or that it was a term for supporters of peace with the Confederacy. There’s a lot we didn’t learn in school. Look up Fernando Wood, for instance.
There was some irony in calling Confederates butternuts, as the Butternut tree generally does not grow in the South.
Copperas is ferrous sulfate, which would bind the dark yellow to light brown color of the walnut shells to the fabric. Both of which were inexpensive, commonly available, and gave fabric a uniform color. A color distinct from the Union Army blue coats.