The phrase "can't cut the mustard" purportedly stems from the idea that mustard is something that adds real flavor or zest to life (the "cutting" refers to the harvesting of the plant). Indeed, mustard yellow is a high risk/high reward color. It can look dated and fusty (think old mustard colored appliances). But it can also look rich, spicy and smoky, especially when paired with grays, blues and even dark red.
Images: As credited above.