Officially it's agate clay. It's so casual, and variable, and unpredictable, it needs a casual name: stripy stuff. You wedge together partly several colors of similar clay, and make pots on the wheel from it. The clays blend and spiral as clay always does on the wheel, but you can see it happening. With enough trimming, scraping, sanding of the throwing slip, the colors show up separately on the pot. I've just been using a dark and a white stoneware for this and getting blond and brunette variants. With stains to color white clays, you can have all sorts of color mixes. That's a mess to make, and, I think, can easily become garish.
I think I prefer a more even clay mix than that, but it's interesting.
It's OK with a clear glaze, but definitely wonderful unglazed.
Dark, or light.
And the stronger color contrast, the better. Do I prefer blonds?