As far as I know a Love Knot is a style of hair which was very
fashionable in the Renaissance era.
It is very similar to a bun, but done by separating the hair
into two sections. The one section is twisted into a bun and
pinned, the next section is then twisted around the bun in the
opposite direction.
Presumably a dark red love knot plaited into hair would then
either be a dark red hairpiece used as one section, or simply dark
red hair put into a love knot.
Hope this helps!