Gradus Group's Vello White Balance Card Set for Digital Photography — $13
We are spoiled in this age of digital post-production for photography. We can apply filters in our cameras, online before posting, or while sitting at a computer or iPad. We can click on a pull-down menu in our photo-editing software and select some filter that accommodates for the color space when the image was taken: a cloudy or bright day, or under different kinds of artificial light. But these are aren't accurate.
At the same time, all professional photo-editing software has the capability to correct the white balance of an image based upon a color "target," true white or neutral 18 percent gray. But how would the software know what's actually white or gray? By your using a target or a white balance reference card set. This Vello card set is inexpensive, easy to carry around and comes on a lanyard, which improves the chance that you won't lose a card — you may lose all of them, but that's a different issue.
The cards are 3.25-by-2 inches. Easy to store.