The colors you are talking about breeding together are an extremely common combination. It is perfectly acceptable. For example, two Blues bred together will result in Black, Blues, and Silvers. Chocolate is in the same series (just the addition of brown dilution to the extended Black). Anyway, breeding those colors together is very common practice. There are certain combinations that are less logical. For example, I don't breed extended Black based birds (like those colors already mentioned) to wild-type based birds (Grey, Blue Fawn, Pastel, Saxony). I also do not normally breed birds with the Runner pattern to Black or wild-type (for example, Fawn and White, Pencilled, Blue Pencilled). HTH, this is the basic answer to how lots of Runner breeders group colors.