Flounder is an ocean-dwelling flatfish species that is found in coastal lagoons and estuaries of the Northern Atlantic and Pacific Oceans. There are many species that can be called flounder, but the most common species is Yellowfin Sole. The color of a raw Flounder fillet will range from tan to pinkish to snow white. No matter what color your Flounder fillets are when you buy them, they will all cook up pure white. Flounder has a mild and sweet taste to it when cooked.