Format guide
What is inside a 56-byte APGB palette?
An APGB file stores 17 RGB colors in 51 bytes, then the byte 81 and ASCII APGB. The first 48 bytes are four groups of four colors: BG, OBJ0, OBJ1 and Window. The next three bytes are the LCD-off color. Each group is documented from darkest to brightest.
What the checker verifies
It verifies the exact file length and footer, reads every RGB triplet, calculates relative luminance, checks group ordering, flags very close adjacent shades and compares same-index object/background colors. These are proxies, not hardware emulation.
What still needs testing
Display modes, firmware, cartridges, openFPGA cores, game scenes, room lighting and subjective color appearance are outside a browser-only static check. Always test the actual target.
Primary source: Analogue developer documentation. This independent tool is not affiliated with Analogue or Nintendo.