Wasn't Advance 1 based on the S3K source code? I thought I remember hearing something to that effect somewhere here on Retro. If that's the case, wouldn't it be better to make it a hack of S3K? The colors could be handled by redrawing the graphics to use dithering just like the old titles, the goal being to make it look relatively close to the original GBA game when viewed in NTSC composite. Another thing to consider is that the gamma should be lowered on the source GBA colors because GBA games in general had heightened gamma levels to compensate for the non-backlit screen. It'd be good to use Nintendo's 1st party games as a reference, specifically to compare the colors from the Super Mario Advance series to the colors used in the Super Mario Allstars version of those same color palettes and perform the reverse amount of gamma correction on the Sonic Advance color palettes.