Neo Green Hill Stuff:

Palette:                $4A7470-$4A7670
Foreground Art:         $4A7670-$4AF830
Collision:              $4AF830-$4B1AB8
128x128 mappings:       $4B1AB8-$4BE1D8
Level Layout:           $4BE1D8-$4BF1BC
16x16 mappings:         I don't think they exist, as 128x128 mappings are made up by individual 8x8 tiles.
Object positions:       Haven't found them yet.

Palette:
The palette is easy to edit. 2 bytes for every color entry. These bytes are in little endian format, meaning the bytes are swapped. For example, 7FFF(which is white) would = FF7F. Sonic Advance Pal Tool is excellent for finding out the color coding for your appropriate color.

Foreground Art:
The foreground art is uncompressed, meaning you can open it in any tile editor that supports GBA 4BPP, or 4 bits per pixel. 8BPP is optional, if you want to edit the Title Screen art.

Collision:
I'm not exactly sure how this works. But this is uncompressed. Anyways, since nobody knows how it works yet, or what the collision definitions are, editting this will be a bitch.

128x128 mappings:
The 128x mappings are built up from 8x8 tiles. Each definition of an 8x8 tile is 2 bytes in little endian format(swapped). For example, if you wanted the right petal of the animated flower, it would be 0100, instead of 0001. I'm not sure how palette switching/flip/mirroring works yet.

Level Layout:
The level layout is extremely similar to Sonic 1, where the layout goes horizontally until it ends that row, then starts the next row. The difference between Sonic 1 is that it doesn't appear to have a 'limit' on the tiles for each row, that I can find at least. The layout starts off with BG 1(as labeled in VBA), then goes to BG 2, which I call the scenery layer. This layer isn't solid at all. Then it comes to BG 3, which I call the solid layer, as every collision here is, well, solid. Well, here's a tile list.

00 - Blank
01 - Top of a wooden railing
02 - Top of a grind railing
03 - Another piece of the grind railing
04 - Top of a palm tree
05 - Full wooden railing
06 - Left piece of a wooden support beam
07 - Right piece of a wooden support beam
08 - Blank
09 - Blank
0A - Blank
0B - Top right corner of a loop
0C - Piece of a grind railing that connects it to the ground
0D - Continuance of a grind railing
0E - Palm tree stem with a coconut next to it
0F - Palm tree stem without a coconut next to it
10 - A support beam that sticks out of the sand(left)
11 - Blank
12 - Blank
13 - A support beam that sticks out of the sand(right)
14 - Blank
15 - Blank
...
34 - Straight Sand
35 - Sand that slopes up going to the right
36 - Top of the slope



NOTES:

Offset $4BF1B0-4BF1B4 Appears to load the 128x128 mappings.
Offset $4BF1B4-4BF1C0 Appears to be coding regarding animated art. a few bytes control which tiles are loaded, and a few control the speed of the animations.



Notes2:
Start of Solid Layer in NGHZ 1(that's where you start at least): 004BEB68