What color is ground for the tail lights for a 98 S-10?

It should be black:

If not black, black with a yellow or beige tracer.

I hope this helps?

Refinish King

Haynes 24071 shows yellow and brown (2 wires) tail/stop turn and black return

General exterior body lighting, tail lights, and low beam circuit for GM cars is brown. Left rear and right rear directional/stop circuits are a second color to the socket and the left is not the same color as the right. Pretty much all plain black wires are always a ground on 12 volt systems.

jmnluna is pointing you right by referencing a repair manual that includes wiring schematics.