if you look through past jam entries (there are links to these on the jam page near the bottom) you’ll probably get an idea of what is allowed. i believe the original Game Boy could only display the same 4 colors (which appeared as shades of green) and no other colors. so if you want to be as authentic as possible, find a game boy palette (there are more than one because the exact shades of green are kind of debatable) like this one that Antigue Goose posted above. but any of the other palettes linked to above are also acceptable because the rules are any 4 colors, not just what the GB had.
as an example, for my past entries to this jam i’ve used palettes that use 4 different hues rather than them all being the same hue (green) like the original Game Boy. this is allowed, it’s just less authentic.
Snack Slugger,
Galactic Pizza,
Worm Life