Sometimes the doctrine of signatures got things right.
The doctrine of signatures is the idea that plants show what human ailments they're good for, in one or more attributes. Let's face it, it's very last-last-last-last-last century. That's the 1500s, in case you lost count.
The color yellow was used as a signature for jaundice: if a plant part is yellow it's good for the yellow skin you get when the liver's gall production goes wonky.
Two plants which were used for jaundice because they're yellow, and which worked, are barberry (Berberis spp.) and yellow dock (Rumex spp.).
The bright yellow barberry root (and the bright yellow lower bark of barberry stems) helps a sluggish liver work better: it's bitter, it's a bile stimulant, it'll help your protein / fat digestion.
The yellow color in Berberis root and bark (and a few others: Mahonia root, Coptis root, etc.) comes from the alkaloid berberine. Berberine is bitter, and berberine-containing plants help sluggish livers because they contain berberine. Barberry root itself is just a nice bland inactive package around the single constituent berberine. Take away the berberine and you have nothing left, except possibly astringency.
Berberine doesn't change color, so the various roots and barks which are yellow because of berberine will retain their color on drying. They'll be a bit paler, but that's just the difference between wet and dry.
Barberry root or bark that isn't yellow won't help your underactive liver: no berberine to speak of.
Mahonia root works just like barberry root, though it does have a few more nuances.
Coptis makes thread-like tiny root mats, and I haven't used that. It'll work the same as barberry root, too, though, and so will its leaves.
Golden seal (Hydrastis canadensis) contains berberine as well, but that's far too expensive a plant to use for its berberine: it's normally used for its far more fragile constituent hydrastine. Except that golden seal is endangered in the wild: if you use it make sure that you use organically grown herb, not wildcrafted.
Yellow dock root (tap-roots from stressed Rumex spp.) doesn't contain berberine; it's yellow only if the plant has been fighting for its survival. On damp meadows you'll find grayish-white roots; they are simple astringents, they're not "yellow dock". Pick it in gravel or sand, there you'll find strong root.
If the root of yellow dock is indeed yellow it's bitter and has that strange harsh taste you get from the larger Rumexes. If dried slowly, on sheets or on paper, the root will change color to a dark brown; if dried in a dehydrator it'll keep some of its yellow color. Yellow dock, too, is a herb for the sluggish liver.
I expect that the doctrine of signatures is still hanging on because there actually are herbs which work according to their signatures.
But really, there's no telling just how many yellow plant parts they tried for jaundice and discarded, as they didn't work for jaundice after all.
The same goes of course for all the other signatures.
You can use the doctrine of signatures as a mnemonic, a way to help you remember things. I wouldn't rely on it to find new uses for plants, though.