As with SNES and SEGA 32X, GBC colors are in 15-bit BGR format. Each color is 2 bytes, with the most significant bit (8000) unused. It's best to view the value in binary format, so that you can see the individual RGB Values for each color. The bit layout is as follows (although it will be byte-swapped; 03FF will appear as FF 03):
X|BBBBB|GGGGG|RRRRR
Examples:
0|00000|00000|11111- 001F Pure Red
0|00000|10000|11111- 021F Orange
0|00000|11111|11111- 03FF Yellow
0|00000|11111|10000- 03F0 Lime
0|00000|11111|00000- 03E0 Pure Green
0|10000|11111|00000- 43E0 Aquamarine
0|11111|11111|00000- 7FE0 Cyan
0|11111|10000|00000- 7E00 Azure
0|11111|00000|00000- 7C00 Pure Blue
0|11111|00000|10000- 7C10 Violet
0|11111|00000|11111- 7C1F Magenta
0|10000|00000|11111- 401F Rose
0|11111|11111|11111- 7FFF White
0|11000|11000|11000- 6318 75% Gray
0|10000|10000|10000- 4210 50% Gray
0|01000|01000|01000- 2108 25% Gray
0|00000|00000|00000- 0000 Black
And if you want to convert from 15-bit BGR to 24-bit RGB, just follow this example:
0|00100|00110|01101- 10CD
Binary->Decimal*8=Value
01101->13*8=104 Red
00110->6*8= 48 Green
00100-> 4*8= 32 Blue
The outcome should be a shade of brown.
If you don't want to follow all of these steps, you can download SNES Palette Editor
here.
EDIT: Sorry about that, Nightcrawler; I didn't realize that it had already been uploaded. The last time I searched for it here, I couldn't find it.