White balance presets on modern digital cameras work well in most situations. However, there are times when no setting seems to produce acceptable colors. This is when a white balance aid comes in handy to allow the photographer to set a custom white balance value.
There are two main types of white balance aids: white balance cards and white balance lens caps. Both perform the same purpose.
White balance cards
White balance cards are actually glorified sheets of paper or plastic cards. They are, however, completely white, something that cannot always be said of paper. By pointing the camera at the card while making sure it is evenly lit by the ambient light source, a correct white balance evaluation can be performed.
White balance lens caps
A white lens cap serves two purposes. The first one is obviously to protect the lens when not in use. They have translucent caps that are used to set white balance. Simply put the cap on and point the camera at the light source. Custom white balance should be very accurate.
Some caps have replaceable white cards, which attach to the inside of the front cover.
It is worth noting that custom white balance can also be set by pointing the camera at a white surface, such as a wall. But not all surfaces are actually white, even if they might appear as such to our eyes.