There are several synonyms for the term "russet-backed" that you can use in your writing. One way to describe the color is "reddish-brown," which is a shade that resembles both red and brown. You can also use "coppery" to describe a metallic reddish-brown hue, or "auburn" to describe a darker shade. Additionally, "mahogany" refers to a dark reddish-brown color that is often associated with fine furniture. "Chestnut" is another option that suggests a rich brown color with a hint of red, while "rusty" implies a more orange-tinted shade. No matter which synonym you choose, you can effectively describe the vivid and warm color of something that is russet-backed.