|There is a significant absence of red light just after sunset on a cloudless evening. Because of this, the color of the light may be outside the camera's range, and therefore the camera's ability to correct. In order to set a correct white balance, the gain in the camera's red channel would need to be increased to its maximum or beyond. This situation may make setting a correct white balance impossible, whether using an ExpoDisc or another custom white balancing aid. When this condition occurs some cameras may display a warning message such as "no good," "over," "under" or "out of range." In certain cases the camera will default to setting the white balance to its minimum or maximum value which may be insufficient to adequately balance the color in the image and result in a color cast in your pictures.
"I've used the ExpoDisc even at times when you're not supposed to correct the light...you can see the resulting image [second image] after calibration with the ExpoDisc. It's not what I saw with my eyes, but sometimes photography is also about what we feel could be there."
- Jose Antunes, Foto Digital (www.fotodigital-online.com)