Why we should say "groene jas" while we should say "groen boek" and not "groene boek"?
Jas is a de-noun, while boek is a het-noun.
I'm trying to distinguish it myself.
You are correct because the definite article is used. That is why "een bruin hoed" is also correct.
I thought ''jas'' was a het-noun..!
Nope, it's de jas.