There isn’t one reliable “correct” white balance for lightning. Its apparent color can vary with the strike itself, the clouds, ambient city light, and the overall scene, so assigning a single Kelvin value usually isn’t very useful.

The safest approach is to shoot RAW and adjust white balance afterward. That gives you freedom to decide whether you want the lightning to look neutral or to keep some of its natural blue/purple appearance. Auto WB is fine as a starting point, and presets like Sunny or Cloudy are creative choices rather than universally correct answers.

In practice, the surrounding light often affects the scene more than the lightning itself—for example, streetlights can push clouds orange while the bolt appears purple or blue. In post-processing, use a neutral area in the frame, if available, to guide correction, then fine-tune by eye for the look you want.

So: shoot RAW, don’t worry too much about finding a perfect in-camera WB, and treat white balance for lightning as an artistic decision based on the scene.