I use the deck of cards analogy a lot. While not perfect, it gets the thought across well enough. If you are selectively breeding, you are trying to get the modifiers to line up into a "deck." With each generation, you get closer to your goal. If you add new blood at that time, the "cards" get scattered again and you are back at the beginning again.
If you just want to have a good colony without selectively breeding, there's nothing wrong with adding new stock. However by doing this you can increase your odds of wilds (or poor coloration) if the colors aren't the same in genes or variety, as well as risking deaths because of different bacteria being introduced. It's just a necessary risk sometimes.