There isn’t a fully objective, perception-proof way to do this. Sunset color is strongly affected by human visual adaptation, changing ambient light, display conditions, and the fact that the sun’s color shifts continuously as it sets.

The practical approach is:

• shoot RAW
• avoid auto white balance
• set white balance manually and adjust it to match your memory/onsite view
• evaluate on a calibrated display, ideally under neutral viewing light

If your goal is to preserve the warm sunset color, don’t white-balance from a sunlit gray card, because that tends to neutralize the warmth you want. A better method is to review and adjust WB while you’re still in the same light, or fine-tune the RAW later on a calibrated monitor.

As a starting point, daylight is about 5780 K before sunset, and during sunset you would generally lower the color temperature setting to keep the scene looking warmer. Because the light changes minute to minute, there is no single exact value.

So: capturing the setting sun “accurately” is less about measuring one objective color and more about careful manual white balance and consistent viewing conditions.