Yesod is assigned the Moon, and the attribution is central to its symbolism: as the moon receives the light of the sun and reflects it to the earth, so Yesod receives the influences of the whole Tree, concentrated through the solar centre of Tiphareth, and transmits them to Malkuth, the material world. In the Golden Dawn system of 1888 and in Liber 777, Yesod's colour is violet, and it corresponds to the four Nines of the tarot suits, the nines being cards of fullness and completion just before the final tens.
The lunar pairing underlies the Western-esoteric reading of Yesod as the sphere of the astral light, of dream, imagination and the subconscious, the subtle level nearest the physical. As throughout, the sefirah Yesod, its name Shaddai El Chai, and its role as the foundation and channel of the divine flow are Jewish Kabbalah, whereas the mapping onto the Moon, a tarot rank and a magical colour scale is Hermetic Qabalah, the Victorian adaptation. The distinction is not pedantry: it separates a theology of divine emanation from a Western system of magical correspondence.