The GBA added colours to black-and-white GB games. So did the gameboy colour. When you turn it on while holding different button combinations, they choose a different colour theme to apply.
That's true but as far as I know, this game was made with the default green theme. But what VBA is showing is much more than than a single pallet:
Well that is unexpected! I can't explain it but enjoy your colour version then! :-D
It fits very well!
Just researching for a different game I want to make, and stumbled across this which may explain some things?
"For dozens of popular Game Boy titles, the Game Boy Color has an enhanced palette built in featuring up to 16 colors - four colors for each of the Game Boy's four layers. If the system does not have a palette stored for a game, it defaults to a palette of green, blue, salmon, black, and white. However, when the user turns on the system, they may choose one of 12 built-in color palettes by pressing certain button combinations (namely a direction key and optionally A or B) while the Game Boy logo is present on the screen."
On the Gameboy Advance page, they also say this: "Game Boy games can be played using the same selectable color palettes as on the Game Boy Color."