Leather dyes

[Martin Moser]

What about other colors than black, like Red, Green, Yellow? How did they achieve these colors?

These colors existed, they are mentioned in Lau, Otto: Schuster und Schusterhandwerk in der griechisch-römischen Literatur und Kunst. Bonn 1967, together with white (alumn tanned leather). Earlier Egyptian finds come with green and red leather, a Roman/Jewish 2nd century bag has red and black, late Roman/coptic shoes have red and gold (gilded leather). The Germanic belt from Gommern came in (most probably) white and gilded leather.
Red can be done with madder, a yellow of sorts with birch leaves, but generally we know very, very little detail about the exact processes unfortunately :-(
An excellent book on dyes (but from around 1500) is the Liber Illuministarum from the Tegernsee monastery.