Sonic 2 Hacking Guide
Binary - Art Editing

This will allow you to modify sprites and tiles in the game. When dealing with the standard Sega 4-bit format, each byte represent 2 pixels. So, if you have something like 43, 4 would be the first pixel and 3 would be the second. This concludes that art is limited to 16 colors. Remember that sprites are edited horizontally. Also, locations with no picture means that there's a blank sprite at that location.

This is the color palette which shows what value is used for what color for Sonic

050000-?????? - Sonic

050020
050040 - Above 050060
050060
050080
0500A0
0500C0
0500E0
050100
050120
050140
050160
050180
0501A0
0501C0
0501E0
050200
050220
050240 - Beside 050200
050260

050280
0502A0
0502C0
0502E0
050300
050320
050340
050360
050380
0503A0
0503C0
0503E0
050400
050420 - Under 050400
050440
050460 - Under 050440
050480
0504A0 - Under 050480

0504C0
0504E0
050500
050520
050540
050560
050580
0505A0
0505C0
0505E0
050600

065000-?????? - Tails

To edit level art, refer to Brett Kosinski's notes. They explain how to modify the level layout. However, the level layout format is identical to the format used on the level art as well. Here are the locations for the art used for each level.

095C24 - EHZ/HTZ art data
09DB64 - MZ art data
0A4204 - OOZ art data
0A9D74 - MCZ art data
0B0894 - CNZ art data
0B6174 - CPZ/DEZ art data
0BCC24 - ARZ art data
0C5004 - WFZ/SCZ art data

There's another art compression format used. It's real name isn't known, so we called it 'Nemesis compression' after the person who cracked the format. I know only a portion of the format. I don't know the rest.

00BD26 - Intro/credits text
074F6C - Title screen (background / logo)
07667A - Title screen (Sonic and Tails)
078F9E - Side spring
0799AA - Points (appear when destroying enemies)
07B592 - Badnick explosion
07C43A - Selection font
07D22A - Title card splash (E, N, O, Z, red, yellow, and blue graphics)
07D588 - Title card splash (all characters, except E, N, O, and Z)
07DA10 - Level select icons
08BE12 - Silver Sonic

003AB4 - Selection font pointer
008C70 - Level select icons pointer