White Balance allows you to color correct your image per-camera, using color temperature and tint properties. Read more about White Balance here.
The White Balance Picker allows you to select an area of your image that is supposed to have a neutral hue, i.e. a white or grey material.
Shift-Click on the Picker icon to Reset the White Balance to default settings.
Temperature (in Kelvin): Blue to Orange color correction.
Tint: Green to Magenta color correction.
In the Add-on Preferences, you can choose between different Color Temperature algorithms.
In the future and for consistency, the goal is to change the default to the new Blender White Balance tool that was added in version 4.3.
Important: Apply at Compositing is not yet available when using the new Blender 4.3 White Balance algorithm (see Add-on Preferences)
Color Management properties only affect the View Transform of the Viewport in Blender. This can be applied to exported images, but not to RAW formats which don't save any View transform. It also won't affect the Color Picker.
Apply at Compositing applies these properties to the pixel using a Compositing node. The White Balance will be saved in the RAW image, and the Color Picker will return the pixel value that is visible in the Viewport or Compositing editor.
Now that Viewport Compositing is available in Blender, there is no real downside in always using “Apply at Compositing”, unless you specifically do not want it to be saved into the RAW file. Make sure to enable Viewport Compositing to preview the White Balance effect.
Since version 5.5, you can easily add a Color Chart to your scene in order to help you define your White Balance value, using the picker on one of the gray swatches.
The White Balance panel is also available in the Image Editor / Compositing Editor. Its picker is slightly different as it uses a Color Swatch, from which you can access the Eyedropper tool