Straight from the Gamefan Official Guide,
The Appendix Code
The Appendix Code can be achieved by pressing B, X, Down, A, Y, when "CONFIGURATION" is highlighted in the option screen. This will open up the Appendix, a hidden menu with a number of glorious options. Most are self-explanatory, but three are not: BGM, SCENERY, CPU COLOR, and, ANIMATION. Under the first three options, you have three choices: RANDOM, CLASSIC and HUNTER. Choosing CLASSIC will revert all colors or music to the original Darkstalkers settings, even replacing the original intro; HUNTER (from "Vampire Hunter," the Japanese name of Night Warriors) are normal NW settings, and RANDOM will select between the two randomly. FULL ANIMATION will give you 100% of the arcade version's animation frames in same character VS same character matches.
Notes: As it's implied, the full animation option only affects mirror matches. Animation frames will remain cut in other matches excluding those.
8-Star Turbo Speed
In the Option mode, highlight Speed and enter the following code: X, X, Forward, A, Z. You can now select up to eight speed stars! We don't recommend this for actual fighting, though...It's impossible to play at such high speeds!
Notes:A gimmick carried over from the SNES version of Turbo Hyper Fighting no doubt. Even though I entered the code, I never bothered using it (I always play at the default normal speed and even go as far as to disable selecting turbo speed and autoblocking altogether.).
Dark Colors
When selecting your character, press and hold the Weak Punch and Medium Kick buttons for a couple of seconds. Your character's color will change into a special hidden dark shade! Throughout the book, "Dark refers to these hidden colors.
Clean Pause
In the Option mode, highlight Screen Size and enter the following code: A, C, Y, Y, Up. Now when you pause the game the screen won't darken and "pause" won't appear!
Hidden Win Poses
Two characters in the game have hidden, selectable win poses - Morrigan and Sasquatch. Morrigan's are selected by holding three punches or three kicks at the end of a round. Sasquatch's is selected bt holding Up and all three kicks at the end of a round.
All have been certified and work when I tried them on NightWarriors.
Hope that helps.