The port's colours are a little brighter than the original's, because 6-bit palette values were scaled to 255

Symptoms

A frame of the port and a capture of the original differ in almost every pixel by a few levels. The brightest colours are 255 in the port and 252 in the original.

Cause

VGA palettes hold 6-bit values (0 to 63). A tool that turns them into 8 bits by scaling maps 63 to 255. Many games shift each value left by 2 instead, so 63 becomes 252, and the screen shows those values. Hardwar's palette loader does this with a shl 2 for each component before it sets the colour (observed, pal_apply, 0x4aaca8). Its palettes also set only 248 of the 256 colours.

What to do

Read the palette loader and use its rule. GameFrame's rawimage and catalog take --dac shift for games that shift, and --dac scale (the default) for ones that scale. The catalogue fills a palette with fewer than 256 colours with magenta, so unset colours show. For a pixel-exact interface the port's palette code must use the original's rule; record it in the palette's format document.

./gameframe run catalog game derived/catalog --palette PAL --palette-offset 4 --dac shift
./gameframe run rawimage dump.bin frame.png --mode linear8 --palette PAL --dac shift