Its called a shiny it occurs 1/8146 (i think) out of every Pokemon encounter
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these different color Pokemon are called shiny Pokemon, each Pokemon has a double that has a different color, like the "yellow magikarp", if you want one, use the old rod over and over again until you have a shiny magikarp.
A yellow Magikarp is a "shiny" Magikarp. The odds of finding any Shiny Pokemon is 1 in 8,192. Unless you use an Action Replay code, it will be VERY difficult to find one.
You catch Magikarp on route 12.
magikarp.....
catch a magikarp and evolve it