Fray Luis' translation of tsammah as hair-locks lacks medieval precedents among Christian exegetes (even Nicholas of Lyra).
Rationale for his Spanish translation of tsammah as hair-locks in Song of Songs 4:1
But even if tsammah could mean "vulva," as Jerome certainly believed, it also meant hair or hair-locks according to the Hebrew experts.
The former supposes an "adjectival" use of melekh as a type of purple ("royal purple," literally, "the purple of a king" where melekh is the second part of a construct), whereas the latter's use is strictly nominal, the hair-locks being compared to "a king captive in the tresses.
Graetz (1871, 154-55), who may have been aware of Rashi's comment, dismisses the exegetical readings of tsammah as veil, hair-locks, or braids, and also translates it as a band, "Binde," to hold the hair (indeed, in his translation, [LANGUAGE NOT REPRODUCIBLE INASCII] begins a new clause: "deine Augen Tauben.
Sforno ignores the latter phrase but offers a brief gloss to explain why the hair-locks are like purple: