Is the cart clear? That's how you know it's a GBC game vs a regular GB game.
To be fair, there are LOTS of releases labeled as part of the Game Boy Color line that didn't get a clear cartridge casing. Link's Awakening DX, Harvest Moon 2, Game & Watch Gallery 2/3, Mega Man Xtreme and Pokemon Gold/Silver are just a few examples. Some of those were released late into the life of the GBC and right up to the release of the GBA.
In the case of Pokemon Yellow, I think it was more in line with the games that were "Color Enhanced" for the Super Game Boy that was released in '94. The packaging on Yellow shows that it was compatible with the GBC but I feel like that's a bit of a misnomer as ALL GB titles were compatible. The game just happened to be released around a month of so of the GBC so it kind of made sense to include that on the back of the packaging.
At any rate, for me the bar on the front says only Game Boy so I think regardless of how many colors the game has, that's the system it should be classified for.