Sienna is a colour: a warm yellowish brown, like dry earth in the sun, or like the clay painters squeeze from a tube. The name comes from the Italian city of Siena, where that earth was dug. Painters still buy raw sienna, which stays more yellow-brown, and burnt sienna, the same earth after heating, which turns redder, more like rust. When someone just says sienna, they usually mean that yellowish brown, not the red. You can use it as a noun (a wash of sienna) or as an adjective (sienna walls). It lives in art class and in careful colour talk. You would not call dirty shoes sienna unless you were being a bit poetic.