Auto White Balance (AWB) and Custom White Balance solve the same problem differently.

AWB makes an educated guess for each photo. The camera analyzes the scene—often giving more weight to bright areas, and on some models comparing the image to built-in scene/color data—to decide what should look neutral. Because it is guessing from the photo content, it can be fooled by unusual lighting or scenes that don’t contain reliable neutral tones. Fluorescent light is a common case where AWB may miss and leave a yellow or green cast.

Custom White Balance uses a reference image that you provide, ideally of a neutral gray or white subject lit by the same light as your scene. The camera measures that reference and calculates how to balance the red, green, and blue channels so that the reference becomes neutral. It then applies that correction consistently to your photos in that lighting.

So AWB is convenient but less precise; custom WB is more accurate because it measures the actual light instead of guessing. For best custom results, use a properly exposed neutral gray or white target and avoid clipping the whites.