None. The W3C/CSS named-color set has no entries beginning with X, the same as E, J, Q, U and Z.

Xanadu (#738678) is a muted gray-green commonly associated with the ornamental plant once known as Philodendron xanadu (now Thaumatophyllum xanadu), whose foliage carries this calm gray-green tone.

Both come from the Greek xanthos, meaning yellow. Xanthic (#EEED09) is a vivid lemon yellow, while Xanthous (#F1B42F) is a deeper golden yellow.

Very few color names begin with X — mostly the Greek xanth- (yellow) words plus a couple of modern brand colors like Xbox Green and Xiaomi Orange.