Horseradish substitute in creamy chowder
If the wasabi is pure wasabi powder, it's only a pale yellow color, not neon green like wasabi you get at a sushi restaurant. A lot of wasabi actually has food coloring in it. Similarly mustard powder is bright yellow... but in either case if you only use a bit of powder (which can be quite strong) it might not impart a lot of color on the final dish.

If the soup needs to be spicy you could try ginger. I also see here you could try black radish, which does have a very spicy taste but apparently it requires advance preparation.