Pokemon in Stadium can have a different colour without being Shiny, though. You can change the Pokemon's colour by giving it a different nickname.
Originally Posted by http://www.pojo.com/videogames/Stadium/tips.html
Alternate Pokémon Colors
Some of the Pokémon that appear in Pokémon Stadium have different colors.
The secret to the color variations lie in the names you give your Pokémon as
well as your trainer ID number (assigned to you in Pokémon Blue, Red or
Yellow, whenever you start a new game). Well there are a few secret names
that change certain Pokémons’ color. These are a few naming tricks that
result in different color variations:
· The whole name is lower caps (i.e.: poliwhirl)
· Reverse the order of the syllables (i.e.: DRILLBEE)
· Abridge the names (i.e.: Pika)
Originally Posted by Bulbapedia
In Pokémon Stadium and Pokémon Stadium 2, Pokémon with a nickname can be colored slightly differently, whether or not they are actually Shiny.
It's a pretty cool little add on. But I didn't think it also happened in the Gameboy games. Especially as they weren't made for Gameboy Color. But if you've noticed it in Yellow, that's pretty cool.