Not a Javalin....totally different fish.
BUT..... I do know there have been many hybrid bream found on the south coast of NSW and even as far north as Sydney.
There is evidence that black bream and Yellow Fin have cross bred and I do believe the 2 fish in the pic above show a greater difference than simply living in fresher or muddier water.
Its quite true that fish often change their colour and general look ( sometimes significantly ) when they live upstream or in a different water condition than their normal coastal environment.
This is especially true for fish that travel upstream and spend a good amount of time away from the ocean. EG. Barra, Bream, mullet, flathead, Jew and even Crabs.
If your posting fish for ID check...... try and extend the dorsal fin and tail because the information to positively ID most fish is in the fin/spike/ray count.
happy fishing
Parkesy