Updated October 8, 2021.
Throughout most of history, threads were dyed with natural materials, usually plants. The colors were soft, not harsh, and the dyes were free of many chemicals that can cause allergies.
With the advent of analine (chemical) dyes almost 200 years ago, all this changed. With chemical dyes, you could produce brighter colors, more colors with less expense and difficulty.
But there are still people and companies that pursue these exquisite colors.
In the embroidery world, my favorites are the naturally dyed wools from Renaissance Dyeing. Follow the link to see their website with pictures of all their colors and examples of work stitched with their products.
Their wool is lovely to use and is very soft. It works best in crewel weight for needlepoint. Treenway Silks also makes a small line of naturally dyed threads. You can also find lots of naturally dyed threads, sometimes these are also listed as plant-dyed.
My suggestion, add this thread to your stash and use it — you’ll love it!