The color is a characteristic that can be easily recognized by sight, however, each Pokémon's color was made official with Pokémon Ruby and Sapphire, where, in the Pokédex, you can arrage Pokémon by this distinction.
Each Pokémon is categorized by only one color, in most cases, the color that predominates over the Pokémon's body, and when the Pokémon has more than a single form, all the forms are categorized of the same color, it can be a neutral or a common color.
This option was omitted in Generation IV, however it came back in Generation V and has been present since then.
The ten existin colors are red, blue, yellow, green, black, brown, purple, gray, white and pink.