choose an S ROM file or a C ROM pair from a game's romset in the file chooser above
Neo Geo ROMs are usually bundled in a zip file. Unzip a game and you'll find a bunch of files. You are interested in S files or C files.
201-s1.s1in Metal Slug or
019-s1.romin League Bowling
019-c2.romfor League Bowling, or
RBFF1_C2.romfor Real Bout Fatal Fury.
Bigger games will have more C ROM file pairs. Real Bout Fatal Fury has C1, C2, C3, C4, C5, C6, C7 and C8 ROMs. You can grab any pair, as long as they go together. C1 and C2 go together, C3 and C4 go together, and so on.
The data inside C ROMs is for the main sprites of the game: the characters, backgrounds, bullets, etc. The data inside the S ROMs is for the fix layer. This layer is drawn above all sprites and it never scrolls. So S data is for things like the current score, number of lives, etc. The "HUD" if you will.
The sprites are stored separate from the color palettes. There's no good way to get the color data from a ROM without running the game. So an alternating blue/yellow palette was chosen as it allows the different colors to stand out.