In Corona Renderer it is possible to use the Corona Color Picker to set automatic white balance for your image directly in the VFB:
1. Add the Tint to the Tone Mapping settings (+ > Basic (Tint) )
2. Click on the "Color tint" swatch in the VFB - the Corona Color Picker should pop up
3. Click on the "Pick inverse" button (icon on the bottom of the color selector window, next to the regular "pick color" button)
4. Left-click on any pixel in the rendered image - it will be considered neutral gray
5. The color tint will be adjusted automatically and applied to your image
Note: it is also possible to set the white balance based on more pixels than just a single one. To do it, select the desired option from the "Point Sample" dropdown list next to the color picker buttons.