There are specimens of the green anaconda, E. murinus, with a reddish orange coloration on the sides, belly, and head. Some people call these "high orange" greens. I have seen both large and small specimens with this coloration, so I would say that they can retain this color into adulthood. Most of them appear to originate from the Orinoco River basin, or the northern population of greens in the north and northeastern area of South America. Really nice ones are quite rare.
On a further note, I still believe that the orange postocular stripe seen in many green anacondas is highly indicative of specimens from the northern section of the greens range, basically outside of the Amazon basin.
Kelly