first, the food that you feed can actually determine coloration. When i was in 8th grade, i set up like 9 seperate tanks with just feeder goldfish. Everything was exactly the same except for the food i fed them. For tank A, i fed regular goldfish flake. For tank B i fed a tropical fish food not meant for goldfish...more for stuff like neons, angels, etc..stuff that has color unlike goldfish. Tank C was fed some marine flake i think lol. I cant really remember that third one tho.
Anywho, each tank had 6 goldfish in them. After 6 weeks, the goldfish in the tank that was fed goldfish flake remained exactly the same with no different colorations. Then the fish in the second tank that was fed with tropical food...they all had weird black and orange patterns after 6 weeks..pretty much like koi do. The third tank didnt do so well, i'm guessing cuz the food wasnt inteded for freshwater fish even lol. But all fish were the exact same fish with all the same parameters, amount of food, everything..just dif. foods...and there was a very noticeable affect on the colors.
But anyways, gold fish and neons and all other tropical fish should not be in the same tank to begin with...goldfish are colder water fish, where tropical fish are..well..tropical water fish lol, meaning warmer.So if you've got the temp set for goldfish, that could have killed off some of the tropicals, and vice versa.
oh, and welcome to aquarium advice