When people think of butternut today, they are apt to describe it as a light, yellowish-brown. The origins of the term butternut are more complex, because it originally encompassed a color, a type of homespun cloth, and a people. The term originated in North. As numerous Southerners moved to southern Illinois and southern Indiana in the mid-nineteenth century, the native Northerners called them “butternuts” for the butternut-dyed, rough homespun cloth they wore. Later, during the Civil War, the they called Confederate soldiers “butternuts” using the same criteria: Southerners wearing rough-textured cloth of the characteristic light brown color. The nickname applied regardless of whether the color had originally been brown from walnut hull dye, or had been dyed gray at the mill and faded to brown. Equally so, it would have been applicable to homemade citizens garb, or to factory made uniforms, as long as the fabric's texture resembled "homespun." Southern-made cassinets, jeans and satinets might all have resembled homespun to a Northerner.
When considering butternut as a color, which the term eventually morphed into by the end of the war, it was a light tan to a light brown color, depending on how the fabric had been dyed. The original butternut color was derived from Northern home dying using walnut hulls of the white walnut tree, commonly known also as the butternut tree. The white walnut, or butternut tree’s botanical name is Juglans cinerea, and its range includes eastern North America from Canada, southwards to northern Alabama, and westwards to Minnesota and northern Arkansas. It is absent from most of the Southland. Northerners used butternut bark and nut rinds (hulls) to dye cloth to colors between light yellow and dark brown. The more concentrated the dye, the darker the resulting color. None used butternut dye used commercially; they used it exclusively for homespun cloth (hence its association with homespun). This white walnut, butternut dye rendered fabrics a light to medium, yellowish brown color. adolphusconfederateuniforms.com/basics-of-confederate-uniforms.html
----- In my confederate units I try to paint the officers in more official outfits. Lower ranks get half gray jackets, half butternut. I give about half the light blue trousers--captured from the Yankees---due to uniform shortages; the rest a mix of browns and grays. If you're painting the Army of Tennessee units they're far less likely to have uniform colors than their cousins in the Army of Northern Virginia.
My painting miniatures book says; Vallejo Beige brown(875) 80%, Hull Red (985) 20% for base coat Then 1st highlight, base coat70% plus Tan yellow (912) 30% If you want a second highlight, 1st highlight 70%, plus more Tan yellow (912) 30% All these are from Vallejo Model colour range.
Some friends of mine are 'costume recreationists' and 'home spinning and weaving and dyeing people'. Have read some of their books. My thots:
There a lot of plants found in the Southern US that can be used for dyes to produce light bronw / tan / yellowish cloth. (check the Smithsonian's publications catalog for this.)
If I were painting up another rebel army, I'd go to the 'craft paint' racks and get every variety of dull yellow, tan, light brown, and light grey they have.
Most of the uniforms for today's Union reenactors come from the same 'dye lot' - there's not much demand these days for heayweight 100% wool cloth in dull light blue. Back then, with lotsa woolen mills and dye houses in America and Europe making cloth for the Union army, and the effects of 'field maintainance' on the result, the Yankee troops are going to be in varied shades of indigo. Note that if you're dying with indigo, and 'overboil' it to 'stretch' the dye, the resulting cloth will have a greenish tinge.
While not doing ACW, I would agree with jdesmond's suggestion. My Romans and other nominally uniformed troops get varying shades of their nominal color. Even if the dye was standardized differing issue dates and exposure to weather and being washed will alter the coloration. Factor in differing sources for the cloth and the variation inherent there and you might not have two the exact same shade in a unit.
Addendum - just look at the labels on commercial clothing produced with vegetable dyes using traditional methods that explicitly state that variations in color are the norm.