Interesting. 'White balance card' - there are many on the market, some true neutral or close to it, some far from it. Some people use index cards, and the bluing agents used in paper can lead to incorrect WB, especially in sunlight (due to fluorescence). Some people use gray cards, which while the luminance is QC'd to 18% gray (they're an exposure tool), the color is not QC'd and varies from lot to lot.

Would these variables affect your results, and if so, do you have a recommendation for an appropriate WB card?