I use a black sharpie. If you look at a baby bass, the line is irregular, like a seismograph. I do my entire baby bass paint scheme, and then use the sharpie to add both the heavy mid line, and also some more irregular blotches and shapes above it up toward the back.
If you look at a few photos of largemouth bass, you'll see they have the same kind of face stripes as smallies, only less pronounced. That's another feature you might want to add.
Just remember a baby bass is trying to survive, so his colors are meant to blend in with his surroundings.