Well, all this talk about my kyusu made me dig it out and give a try with some slightly old Fukujyu sencha. Actually came out pretty tasty
Chip, it appears that the ridges are formed by "engraving". In other words, the light colored clay (slip or possibly a glaze) is added and then scraped away, revealing the darker color underneath (the same color clay as used for the spout and handle).
Glad to hear the Fukujyu Japanese green
(?) came out tasty for you in your Kyusu ... I can't seem to remember this selection however? Any info?
I have looked at some slipped Kyusu-s here. It seems like a "stencil" type process. Tape is applied (in the case here, Momiji leaves," the slip was then applied, then the leaves were removed leaving the contrast of smoother under layer and rougher raised layer. This created a multi dimensional layer or depth ... and color contrast. Here the slip is green.
Perhaps this is how they also created the striping? But I only pretend to know what I am talking about.
Hopefully some potters will jump in. The only explanation I ever heard was in broken Japenglish.