Undoubtedly, one of the most critical aspects of digital photography or videography is the consistent control of color. The color values recorded by a camera's sensor need to be matched with the color palette of a monitor, and the eventual output device as well.
The Color Balance Lens (CBL) is a 2-sided disc with a gray side and a white side, to handle your needs of color balance and proper exposure alike. You use the white side for color balancing, and after the white balance is achieved you use the gray card for an accurate exposure reading.
Adjusting color balance with digital cameras varies, but in every case you hold the CBL in front of your camera and either make an evaluation or take a white-balance-reference picture. You should assure that the subject's dominant light source is also falling upon the CBL. Thanks to its complex, varied collection of prisms and lenses upon the 'white' side, the CBL grabs a large mixture of the light source and directs it into the camera during the white balance evaluation.
You are not reading light reflected off of subjects, and in this way the CBL is consistently more accurate just as incident light meters are more accurate than reflected light meters. You are not evaluating the white balance from a flat reflection of a gray/white card, which cannot gather as relevant a 'recipe' of reflected light from complex scenes. While white/gray cards or lens-attached white balance devices often yield fine results, it is in the most difficult lighting scenes where the CBL's design shines best.
To use the gray side for evaluating exposure, you simply use the CBL's gray side just as you do a gray card: use your camera's spot or partial metering mode if applicable, and place the CBL's gray side in your subject's lighting source.