Take it from someone who has put up ALOT of money on goldfish, the pricier ones don't stay any truer to color than the ones you find in the petstore. In terms of black, hard water and strong sunlight, along with an avoidance of color enhancing foods are supposed to help maintain it, but in the end, it still rests mainly with genetics. Blues, browns, purples, are supposedly based on the genes responsible for black, which is unstable enough as it is, so they are all also relatively unstable. In the end, most of your goldies will end up exactly that, or white unless they are calicos.