IIRC, Japanese Yellow doesn't support GBC colour. (I'd expect that all your Pokémon would show up in the same colour.) There's 3 explanations I can think of for this:
1. You were looking at it using a Game boy system. Does everything else (besides character sprites) appear blue?
2. It's a glitch. Possibly a Missingno. with Nidoran♂'s art but a different palette. Did it learn the usual level-up moves? Did it evolve at the usual level? Into a Nidorino?
3. You're looking at it under G/S/C/Stadium GS. Shinies can technically be found on Colour-gen games, but only with the usual odds, and you can't see whether or not they're actually shiny. If you trade such a poke to a Metal-gen game, it'll show up as shiny. This is because there isn't a specific flag for shininess; Pokémon with particular IVs are intrerpreted as being shiny by Metal-gen games. Were you looking at it under a Metal-gen game?
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