Hii

[Jaerri]

Hello there and welcome to the forum!
What you mean is some special kind of pokemon. It's a "shiny".
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normal - shiny
Every pokemon has some other colour scheme as shiny. other than that, it doesn't really matter. Like for stats and so on.
Shinies are very very rare to find.
You can breed for them in a special way, called Masuda-Method;
Or fish them via Chain-Fishing;
Or may be soft resetting the intro for getting the starter shiny.
... and many other ways! It can take some (long) time to get what you want for yourself. 
You can keep an eye out in this very nice community too. For giveaways or just ask for it (like you did haha)
You can read up HERE everything about shinys!
have a nice day