"Color palettes used for original Game Boy games
Directional pad Action button
None A B
Up Brown Red Dark brown
Down Pastel mix Orange Yellow
Left Blue Dark blue Grayscale
Right Green Dark green Inverted
When playing an original Game Boy game on a later system, the user can choose which color palette is used. This is achieved by pressing certain button combinations, namely either A or B (or neither) and a direction key while the Game Boy logo is displayed on the screen.
These palettes each contain up to ten colors.[8] In most games, the four shades displayed on the original Game Boy would translate to different subsets of this 10-color palette, such as by displaying movable sprites in one subset and backgrounds, etc. in another. The grayscale (Left +
palette produces an appearance essentially identical to that experienced on the original Game Boy, the inverted colors palette (Right +
inverts the colors from the Game Boy Color's color palettes.
Illustrated color-samples of the palettes for the different key-combinations. Any color crossed out will be present in palette RAM, but rendered as transparent.
In addition, 93[citation needed] Game Boy games have a special palette that is enabled when no buttons are pressed. Any game that does not have a special palette will default to the dark green (Right + A) palette. Notable games that do have preset palettes are Metroid II: Return of Samus, Kirby's Dream Land 2, Super Mario Land, Tetris, and the Wario Land series. The default palettes are stored in a database within the internal boot ROM of the GBC, and not the game cartridge itself.[9]
Super Game Boy titles will work too. Despite compatibility with Super Game Boy titles, the Game Boy Color will not recognize the coloring header for Super Game Boy titles which have a color graphics system of its own.
Hi-Color Mode
A few games used a technical trick to increase the number of colors available on-screen. This "Hi-Color mode" is a mode used by the Italian company 7th Sense s.r.l. among others, and can display more than 2000 colors on the screen. Some examples of games using this trick are: The Fish Files, The New Addams Family Series, and Harry Potter and the Chamber of Secrets."
-Wikipedia.