Auras is a thing that determines a Pokémon's potential, starting from red, then to silver, emerald, then to diamond, and finally to being black. Each aura shows how much potential it has before reaching its maximum level.
Red=30
Silver=55
Emerald=65
Diamond= 85
Black= 100
Trying to rank up one's aura requires a lot of materials just to rank up a Pokémon with a red aura, and even more materials than the last. Getting the materials is not easy. You would have to harvest something from a high-level Pokémon or even an alpha Pokémon, which would slightly increase the potential by ten percent.
Giving recourse to a Pokémon when it is evolving gives a boost of five to seven percent of an increase.
Certain Pokémon will also get an increase in their potential. Steven Shiny Metagross has a higher percentage gain than a normal Metagross, as well as being boring with a strong aura to begin with.
I might add this into the story