In the Southeastern United States a backpack is suspended from the shoulders, while a rucksack is a heavy duty backpack that has extra straps that go around the waist and/or across the chest to distribute the weight. When I was in school, one might call a backpack a "bookbag" but never just a "bag". It's nice to hear the subtle usages in other parts of the world.
In the right context - yes, that's how it's used in Australia from my experience too. For example, amongst school children and their parents it would usually be clear that the "bag" is the "schoolbag", usually a "rucksack".
I suppose whether or not it's an appropriate natural translation depends on whether we should interpret the German sentence as emphasising the specific type of bag, or just the colour. Without knowing that, I think it's better to keep the translation literal, using either "backpack" or "rucksack" in English.