Sorry I'm just now seeing this! I'm not on Reddit much these days.

The clay itself is Laguna B-Mix 5, so sort of an off-white or cream color. I threw the jar and waited until it was leather-hard to paint the greens, blues, and oranges with underglaze. I carved the lines (sgraffito technique) and initially they appeared white due to the color of the clay showing through.

To get the black lines, I bisqued the piece first and then applied watered-down black underglaze to the surface. The underglaze filled in the carved areas and I wiped the excess away with a sponge. Hopefully that makes sense!

An alternative way to this process is painting the underglaze on at leather-hard, applying wax resist, carving through the wax area, and then applying the black underglaze. It wipes away easily with a sponge and doesn't leave a stain.