The Fire Flower's colors in this game are different than those in the original NES version of Super Mario Bros because the NES version uses 8x8 sprite tiles while the GBC version uses 8x16 sprite tiles.

The Fire Flower is made of four 8x8 tiles in the original Super Mario Bros. The bottom two tiles always use the green palette while the top two tiles cycle between the four available sprite palettes.
They had to go about it differently in this game. The Fire Flower now uses two 8x16 tiles instead of four 8x8 tiles, so the top palette can't change without also changing the bottom palette.

However, the Game Boy Color supports twice as many sprite palettes as the NES, so the blue/green/gray from the 1-up Mushroom's palette is used in one of the extra sprite palettes. This extra palette slot also ended up being used for the Poison Mushroom.

Great! This game needs more acknowledgement.