The book’s example is not the best. Green great dragons can absolutely exist, if great dragons were a breed of dragons, like great danes. In that case great would be a way more specific modifier of the noun dragons, as it defines a subgroup of the group of the noun. That’s why it can be closer, because it’s more specific than green, and our brain can definitively switch automatically to get this nuance in language, as soon as we asses that “great dragons” are a thing. It’s not about size and color in itself, it’s just that, generally speaking, size tends to be less specific than color in subgrouping a noun. This is also how we learn though trust: if I find the sentence and I trust who wrote that, I assume and therefore learn that Great Dragons are probably a breed of dragons.