It's been a while (20 years) since I did the technique in production. I only saved a couple of pieces for myself, so I don't have lots of photographic examples. But here's a fairly large (14" tall) lamp done this way.
In this case, what I refer to as the "ground line" -- the first color drip I send down the side -- was the red iron slip. The second line I created with the black slip -- 2% cobalt ox, 3% red iron ox. Finally, I finished by chasing a line of porcelain slip right down the same channel as the black slip.
I discovered early on that the ground slip will dictate how heavy the whole pattern appears. And because the second line needs to be thinner (meaning: "more liquid/more fluid"), you really need to start with a ground that has some body.
The idea is to create parallel vertical lines with the ground slip. The second color of slip should then be able to chase down between two lines, clinging to BOTH lines, and creating a now-unbroken, slip-covered surface to the face of the pot.
If the slip you use for the second line is thicker than the ground line, the second drip will simply cling to one line instead of dripping down evenly between two lines.
The third and final color of slip should be somewhere between the thicknesses of the other two colors, but it can be as thick as the ground. It's most important that it be at least slightly thicker than the second color. If it is, it will chase that second color straight down, leaving a pencil-thin line of the second color on both sides of the third color.
Finally, after making sure that gravity had had it way with all the drips and their downward drift was more or less finished, I created the horizontal lines with a tool I created by taking the needle out of a needle tool and replacing it with a length of B string from my guitar. I created one set of lines with the wheel spinning as it does when I throw, and then I reversed the wheel to create the alternate horizontal lines.
In these feathered pots I used both high-contrast colors (black and porcelain on the same piece), and no-contrast (body colored slip of varying thicknesses with a glaze that emphasizes texture over them). I also quite often left the lines alone -- did not feather through them. The possibilities are nearly endless.
The examples I've shown so far are all under a semi-matte glaze that pleasantly mutes the colors. But if you want the colors and patterns to really POP, they really come alive under transparent and celadon glazes. I don't have any photo examples of vertical feathering under transparent, but I do have these plates that are the same colors of slip -- feathered in a ground -- and under a cone 10 transparent glaze.
Under the transparent, obviously the slip that creates "black" under the matte glaze, appears as midnight blue.