(using rgbds v1.0.0)

The error occurs when providing an existing palette (via -c gbc:existing.pal) that was generated with the --color-curve option. It appears that the colour correction gets applied to the existing palette which prevents the colours in the source image matching.

What I expected to work:

❯❯❯ rgbgfx -C -p withC.pal -- afish.png
❯❯❯ rgbgfx -C -c "gbc:withC.pal" -o afish.chr -- afish.png
error: Failed to fit tile colors [$0066, $0f5f] in specified palettes
note: The following palette was specified:
        [$0bff, $0045]
Conversion aborted after 1 error
❯❯❯ xxd -e -g2 -c8 withC.pal
00000000: 8000 0f5f 0066 ffff  .._.f...

You can see the colours listed as the specified palette are different to the contents of withC.pal.

This seems like a bug and I expected this to work in part because the manpage says the curve applies when generating palettes:

	-C, --color-curve
			When generating palettes, use a color curve mimicking the Game Boy Color's screen.