Getting the most from tannins
Dark, rich tones like the ones you can achieve from the Black Walnut tree are exquisite. They are velvety and deep. One of the reasons we gravitate towards dark neutral tones is that they are so wearable against so many skin types.
We are accustomed to selecting from any color on the spectrum now, but synthetic dyes didn’t exist before World War I. That means, only until quite recently have we switched from natural dyeing to industrial, synthetic processes. Oddly, there seems to be a misconception that natural dyes are pale and fade quickly. I’m here to prove that you can get intensely, deep, saturated colors with raw materials. It just requires time, intensity, and heat.
Walnut husks, leaves, and wood contain tannins and flavonoids, making them ideal for natural dyeing. Because of their tannin level, they can even yield pigment without the use of a mordant. After several experiments of obtaining various shades of brown, I finally cracked the code for getting intense, dark tones. By combining these tannins with small amounts of iron, one can achieve even deeper shades. But the missing piece of the equation was extra time. Long, slow dye baths of at least 4 hours will produce the deepest brown shades. Interesting, when dyeing with black walnut leaves, the flavonoids will adhere to the textile first, and then the tannins will bind as it remains in the dye bath.
Source: The Art and Science of Natural Dyes by Joy Boutrup & Catherine Ellis