Much like ANH's re-use of droids I guess, the same can be said of astro's in the prequals.
The thing is, to me anyway, droids are like cars... how many aqua mid-90's Chevy Cavaliers have you seen? Or what about the black and dark blue ones today? How many dark blue Ford Explorers?
That's one planet... You know there's gotta be a myriad of blue and white R2's too, we just see the one. Some decoes are probably custom, a little more obscure or "rare", but others are probably just run-of-the-mill paint jobs you get off the showroom floor.