Mustard is the blossom of depression. This condition we describe as a dark cloud, suddenly descending upon us for no reason, with unknown origin. It lifts as it comes, on its own – unexpectedly and without explanation. This is a condition of deep darkness, of hopelessness and despair. In its positive aspect however, it brings light and pure joy of life as well as an unshakeable internal peace.
Those who are liable to times of gloom, or even despair, as though a cold dark cloud overshadowed them and hid the light and the joy of life. It may not be possible to give any reason or explanation for such attacks.
"The Twelve Healers and Other Remedies"