I've definitely seen "yellowness" specifically mentioned as an indicator of tudei or at least certain varieties of tudei kava. It's been mentioned here before that Isa stands out as having yellow insides. But I've also seen yellowness in my own noble kavas and have seen it described in regard to other specific noble kavas.
I've pulled some quotes:
"In water, kavalactones form an unstable emulsion. This is the reason why kava drinkers stir up the yellow, oily, and starchy bottom of their bowls often." -The Hawaiian 'Awa Book
"Rootstock color varies from white to dark yellow, depending upon the amount of psychoactive kavalactones that are contained in a lemon yellow resin." -Kava: The Pacific Elixir
"They clean the kava with their knives and polish it with handfuls of coconut fiber. After giving it a final inspection, they bite off yellowish stump and begin to chew. Each is careful to mix in his mouth pieces of the stump itself with one or two elongated kava roots." -Kava: The Pacific Elixir
"Chewers are soon surrounded by pads of green leaves on which sit growing mounds of masticated yellow kava." -Kava: The Pacific Elixir
"(Tanna) Ahouia: The stump of this cultivar (whose name means "yellow") has a very distinct yellow interior, indicating that is is very potent (i.e. rich in kavalactone resin). -Kava: The Pacific Elixir
"(Tahiti) Marea: Root was lemon-yellow on the inside" -Kava: The Pacific Elixir
Then I have these two images that contradict each other, assuming 1-6 on the 2nd image correlate to the 6 major kavalactones.