Two color morphs of St. Vincent Amazon (Amazona guildingii) - Theory

My theory is that the two natural color morphs of the St. Vincent Amazon (Amazona guildingii) are a mutation in the varying production of red psittacofulvin. In the dusky lory (Chalcopsitta fuscata, former Pseudeos fuscata), a yellow mutation occurs that causes the orange psittacofulvin to oxidize only to yellow: https://pmc.ncbi.nlm.nih.gov/articles... I suspect a similar mutation could cause the difference in the St. Vincent Amazon that naturally occurs in both green and brown color morphs. The base color of the wings and body is a structural blue, overlaid with either yellow or orange psittacofulvin production. I demonstrate the difference by using a multiply blend mode in image editing software.