This is the UPDATED information on L-Cysteine production for Pheomelanogenesis, which says nothing about L-Cystenine levels falling and creating Eumelanin as this "guess" (not even a theory) makes no sense published May-Jun 2008
Chemistry of mixed melanogenesis--pivotal roles of dopaquinone.
Ito S1, Wakamatsu K.
Melanins can be classified into two major groups-insoluble brown to black pigments termed eumelanin and alkali-soluble yellow to reddish-brown pigments termed pheomelanin. Both types of pigment derive from the common precursor dopaquinone (ortho-quinone of 3,4-dihydroxyphenylalanine) which is formed via the oxidation of l-tyrosine by the melanogenic enzyme tyrosinase. Dopaquinone is a highly reactive ortho-quinone that plays pivotal roles in the chemical control of melanogenesis. In the absence of sulfhydryl compounds, dopaquinone undergoes intramolecular cyclization to form cyclodopa, which is then rapidly oxidized by a redox reaction with dopaquinone to give dopachrome (and dopa). Dopachrome then gradually and spontaneously rearranges to form 5,6-dihydroxyindole and to a lesser extent 5,6-dihydroxyindole-2-carboxylic acid, the ratio of which is determined by a distinct melanogenic enzyme termed dopachrome tautomerase (tyrosinase-related protein-2). Oxidation and subsequent polymerization of these dihydroxyindoles leads to the production of eumelanin. However, when cysteine is present, this process gives rise preferentially to the production of cysteinyldopa isomers. Cysteinyldopas are subsequently oxidized through redox reaction with dopaquinone to form cysteinyldopaquinones that eventually lead to the production of pheomelanin. Pulse radiolysis studies of early stages of melanogenesis (involving dopaquinone and cysteine) indicate that mixed melanogenesis proceeds in three distinct stages-the initial production of cysteinyldopas, followed by their oxidation to produce pheomelanin, followed finally by the production of eumelanin. Based on these data, a casing model of mixed melanogenesis is proposed in which a preformed pheomelanic core is covered by a eumelanic surface. Source: https://www.ncbi.nlm.nih.gov/pubmed/18435614
As stated above Eumelanin and Pheomelanin are two chemically different compounds. So even if Pheomelanin levels dropped one would not attain Eumelanin through the depletion of sulfhydryl compounds, because they follow completely different pathways of synthesis. This is determined by Genetics, one would have to turn on the SLC24A5 and SLC45A2 genes as a start.
α-MSH/MC1R plays a major role in the production of Eumelanin. MC1R Variants
"Melanocortin 1 receptor is a highly polymorphic protein, and in humans many of the loss-of-function variants are associated with the “red hair color” (RHC) phenotype. Individuals with a dysfunctional MC1R may have decreased eumelanin synthesis leading to fair skin and an increased sensitivity to UV exposure."
MC1R and Pigmentation
"There are two major types of pigment present in the skin, the darkly pigmented eumelanin and the red/yellow sulfated pheomelanin. Eumelanin is chemically inert and is highly photoprotective by absorbing UV radiation and oxidants. In contrast, pheomelanin is much less efficient at blocking penetration of UV radiation into the skin and can promote UV-induced cellular damage by contributing to free radical and oxidative injury. MC1R signaling is a major determinant for the amount and type of melanin pigments synthesized by melanocytes, regulating both basal pigmentation and the UV induced tanning response. **MC1R signaling increases **eumelanin synthesis****, the ratio of eumelanin-to-pheomelanin, and enhances melanosome transfer to enhance melanin deposition in keratinocytes.
Both eumelanin and pheomelanin derive from the sequential cyclization and oxidation of the amino acid tyrosine (Figure Figure33) (Ito, 2003). The first two biosynthetic steps are shared between the two pathways: the conversion of tyrosine to DOPA and then to DOPAquinone by the enzyme tyrosinase. Eumelanogenesis and pheomelanogenesis diverge after formation of DOPAquinone. Other enzymes beside tyrosinase are needed for melanin synthesis including dopachrome tautomerase and tyrosinase-related protein 1. Defects in many pigment enzymes yield hypomelanotic phenotypes such as albinism. Pheomelanin production is dependent upon the incorporation of a cysteine and retention of sulfur after the synthesis of DOPAquinone, which may explain why mature pheomelanin pigments are reddish/yellow rather than dark brown/black as eumelanin is. Although the control of the pigment switch between eumelanin and pheomelanin is regulated by multiple factors including the pH of the cellular milieu and the levels of tyrosinase (Burchill and Thody, 1986; Ancans et al., 2001), the presence of a functional MC1R is required for effective synthesis of eumelanin. Since eumelanin absorbs UV radiation, the more eumelanin the skin has, the more protected it is from UV damage." Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4885833/