Thanks for the comment! I agree that a lot of these things aren't made explicit.
The likelihood of dark cells forming increases as you grow in size and the longer you wait. Reacquiring the dark cells after they split from you is one possible strategy, but that will continue the spread more rapidly. Basically, the dark cells incentive use to grow as fast as possible so that you can defeat the bosses before losing too much mass.
You don't need very much color to leave a cell. I did not make this very clear in the game.
The order you pick up colors does not matter. Do note that the dark cells will have zero chance to form while you're entirely white.