I probably was the one that explained why a search in that field has disappointing results. It was always meant to be checked for foliage of interesting colors, but the data checkbox used to be called "Good growing season color." It therefore got checked for almost every plant because even plain green is a good growing season color.
The name was changed to "Unusual foliage color" in January 2014, but the plain green plants were never weeded out of that category. Good rainy-day project, I suppose.