Ocher de rux is a pigment discussed in Susie Nash's chapter on the Burgundian Courts in Trade in Artists' Materials.
'On a few occasions a vastly more expensive type of ochre called de run (or up) was supplied, priced at 120 and 150d/lb, 24-30 times the cost of the ordinary sort (Table 16, nos. 15, 17). It can be found referred to in other documents of the period: in 1398 Alcherius recorded a recipe in Paris from an illuminator, Antonio de Compendio, in which 'ochre de ru' was specially recommended, mixed with white chalk, as a mordant for gilding. According to Jo Kirby it may have had a particular brownish colour. Its origin is unknown.' p.155-6Trade in Artists’ Materials, Kirby, Nash, Cannon.

Could this be a case of the author using something that was already in the workshop for gilding? Seems unlikely as it is such an expensive choice. Why would he have suggested such a costly pigment for the shadowing layer of a woman's face?

Also Pigment Glossary, Kirby and Nash'Yellow or brownish-yellow iron oxide pigments, deriving their colour from goethite (α-FeOOH) or a similar mineral. Some regions, notably central Italy and France, were the sources of ochres of particularly good, bright yellow colour.The origin of the name over de Luke is uncertain, although both Lucca (Italy) and Liege (Luik in Dutch) have been suggested; over de rose or de run, which seems to have been a brownish-yellow, is similarly obscure.' p.453
I have chosen a bright brown coloured ochre for this, based on these instructions.