First off, this article is about a general trend, saddly not all private breeders fit the following generalization.
I noticed several folks talking about the lack of color in their female guppies.
Below are two pictures of FEMALE guppies from breeder stock that I have here.

Young half black female guppy

2 Adult female yellow platinum guppies
I hope the difference between these fish and what is generally seen in shops is obvious.
GENERALLY, fish purchased from breeders are of a much better quality than shop fish.
This is due to the fact that shop fish are mainly pond bred vs breeder fish being raised where the lesser fish can be removed from the breeding program.
While not as glaring with most other species than guppies, breeder fish generally have better color. They are also ususally in better health.
So if you have the chance, use the various auction sites, join one of the national clubs or try to find breeders close to you.
It is well worth the effort.
Dino