I use Hikari BioGold+ for pellets (40-50% of their diet) but mine had awesome color when I was just using regular Hikari. Since they were babies I've also always fed mussel, crayfish, krill, earthworms, spirilina, crickets. One of the biggest factors is using real dark substrate. When my Red Oscar was temporarily in the 125 g with light sand (while we were setting up his 250 gal) his base color was more that greenish color although his red did stay super red. Once he got into his permanent tank with dark brown sand his base color turned solid black with the red. Made him look a lot better.
They also won't get their bright red until they're usually around 7-8", if your Oscar is smaller than that just feed him good, change lots of water and wait.
Edit: Actually I shoul've said THE biggest factor is keeping nitrates low IMO.