GBDK fadeout for Game Boy and Game Boy Color

fadeout.c

#include <gb/cgb.h>
static const UINT16 bg_palette[] = {RGB_WHITE, RGB_LIGHTGRAY,  RGB_DARKGRAY, RGB_BLACK, RGB_BLACK, RGB_BLACK, RGB_BLACK};
                                    
...

void fadeout(){
  for (i = 1; i != 4; ++i) {
    // 0xFFE4 is 11'11 11'11 11'10 01'00
    // (i << 1) is i*2
    // each iteration the number gets shifted by 2 bits to the right
    // each two bit pair represents a color on DMG
    // BGP_REG is 8 bit and upper 8 bit will be discarded on cast
    // i=0 would be 11'10 01'00
    // i=1 is 11'11 10'01 etc.
    BGP_REG = (0xFFE4 >> (i << 1));
    // Sets palette #0 for Game Boy Color
    // bg_palette is just the start address of the array
    set_bkg_palette(0, 1, bg_palette + i);
    performantdelay(100);
  };
  // don't use wait_vbl_done() until display is turned on again
  DISPLAY_OFF;
}