Color Palettes Overview
Available SNES Palettes
The SGB/SNES provides 8 palettes of 16 colors each, each color may be defined out of a selection of 32768 colors (15 bit). Palettes 0-3 are used to colorize the gamescreen, only the first four colors of each of these palettes are used. Palettes 4-7 are used for the SGB Border, all 16 colors of each of these palettes may be used.
Color format
Colors are encoded as 16-bit RGB numbers, in the following way:
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ignored | Blue | Green | Red | ||||||||||||
The palettes are encoded little-endian, thus, the Red+Green byte comes first in memory.
This is the same format as Game Boy Color palettes. However, the same color will be displayed differently by SGB and CGB due to the different screen gamma!
Here’s a formula to convert 24-bit RGB into SNES format:
(color & 0xF8) << 7 | (color & 0xF800) >> 6 | (color & 0xF80000) >> 19
Color 0 Restriction
Color 0 of each of the eight palettes is transparent, causing the backdrop color to be displayed instead. The backdrop color is typically defined by the most recently color being assigned to Color 0 (regardless of the palette number being used for that operation). Effectively, gamescreen palettes can have only three custom colors each, and SGB border palettes only 15 colors each, additionally, color 0 can be used for for all palettes, which will then all share the same color though.
Translation of Grayshades into Colors
Because the SGB/SNES reads out the Game Boy video controllers display signal, it translates the different grayshades from the signal into SNES colors as such:
| GB color | SNES palette index |
|---|---|
| White | Color #0 |
| Light gray | Color #1 |
| Dark gray | Color #2 |
| Black | Color #3 |
Note that Game Boy colors 0-3 are assigned to user-selectable grayshades by the Game Boy’s BGP, OBP0, and OBP1 registers. There is thus no fixed relationship between Game Boy colors 0-3 and SNES colors 0-3.
Using Game Boy BGP/OBP Registers
A direct translation of GB color 0-3 into SNES color 0-3 may be produced by setting BGP/OBPx registers to a value of $E4 each. However, in case that your program uses black background for example, then you may internally assign background as “White” at the Game Boy side by BGP/OBP registers (which is then interpreted as SNES color 0, which is shared for all SNES palettes). The advantage is that you may define Color 0 as Black at the SNES side, and may assign custom colors for Colors 1-3 of each SNES palette.
System Color Palette Memory
Beside for the actually visible palettes, up to 512 palettes of 4 colors each may be defined in SNES RAM. The palettes are just stored in RAM without any relationship to the displayed picture; however, these pre-defined colors may be transferred to actually visible palettes slightly faster than when transferring palette data by separate command packets.