As said, male guppies only are a good selection. Most fish stores sell the male and females separate, so it's easy to buy only males. Besides, the males are the ones with all the fancy color and tails, the females tend to be plain looking most of the time.
True, but I've seen splendid color on some females. Even the ones I have have been coloring up over generations. Not a lot of solid strains being traded locally, afaik