Do you know if their Soylent recipe uses carrots or other pigmented vegetables? It could be an accumulation of the coloring. (This apparently happened to me as an infant with carrots. Made my face red/orangish.)
The early version contains carotenoids found in pigmented vegetables, at least lycopene found in tomatoes, and alpha-carotene found in carrots. It seems you'd get much less carotenoids from Soylent than just eating one tomato and one carrot per day.
He mentions "not very scientific, but the males in my family have always loved tomatoes." Perhaps that's the explanation and not Soylent, although you get three times less carotenoids from tomatoes compared to carrots so you'd probably have to eat ridiculous amounts of them to become red. Perhaps they love carrots too.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.