The abbey I’m associated with doesn’t specify a liturgical colour for Holy Saturday. It does for Good Friday (red) and Holy Thursday (white).
Likely because there is no Mass on Holy Saturday, and the monks would simply be in choir in their black habits, with the abbot wearing his pectoral cross.
All Offices during the Triduum, except for the Easter Vigil itself, are in the monastic tradition stripped bare (this was the case in the pre-Conciliar Roman Office as wells and is one time when the Roman and Monastic Offices were the same). There is no hymn. The monks cross themselves at the incipit of the first antiphon. No doxology at the end of the psalms, and no closing verse/ritual; the responsory is replaced by the gradual “Christus factus est”. So no vestments either other than their black habits, to go with the sombre mood.
The LOTH has chosen not to carry forth this tradition alas, probably because the Offices are already very short to begin with. Still, it would strike as fitting to signal this uniquely and intensely barren day by keeping clerical garb as simple as possible.
Edit: I checked the ordo of the Canadian Conference of Catholic Bishops and it specifies violet for the Liturgy of the Hours. But that does not apply to the monks who have their own ordo based on that of Solesmes.