Real authentic wasabi is made of 100 percent wasabi plant (Wasabia japonica). It is naturally green, while horseradish (its common substitute) is white or brownish. Buy it at a trusted seller or at a specialty store, to be sure. For wasabi paste or powder, fake wasabi contains several ingredients to imitate the characters of real wasabi. These include mustard, coloring, flavoring, starch, and many more. If you spot these ingredients, then you know what you get.