Palettes

LCD Monochrome Palettes

FF47 — BGP (Non-CGB Mode only): BG palette data

This register assigns gray shades to the color indices of the BG and Window tiles.

Each of the two-bit values map to a color thusly:

In CGB Mode the color palettes are taken from CGB palette memory instead.

FF48–FF49 — OBP0, OBP1 (Non-CGB Mode only): OBJ palette 0, 1 data

These registers assigns gray shades to the color indexes of the OBJs that use the corresponding palette. They work exactly like BGP, except that the lower two bits are ignored because color index 0 is transparent for OBJs.

LCD Color Palettes (CGB only)

Colors on the Game Boy Color are stored as RGB555, meaning a single color is composed of three 5-bit components, one for each of red, green, and blue. Each 15-bit color occupies the lower part of a 16-bit “word”¹:

The color palettes are stored in two dedicated banks of palette RAM (or CRAM for color RAM), 64 bytes each²: one for background/window palettes and the other for OBJ palettes.

The two bytes of each color are stored little-endian, meaning that the low byte comes first. For example, the two palettes shown in the previous diagram would be stored like this:

Unlike VRAM, OAM, or wave RAM, CRAM cannot be accessed directly, but instead through pairs of registers: one “data” register provides a one-byte view into CRAM, while the corresponding “index” register controls which CRAM address the data register is bound to. Much like VRAM, CRAM is inaccessible when the PPU is reading from it, that is, during Mode 3: writes are ignored, and reads return $FF.

FF68 — BGPI (CGB Mode only): Background palette index

• Auto-increment: 0 = Disabled; 1 = Enabled
• Address: Specifies which byte of BG Palette Memory can be accessed through BGPD

Unlike BGPD, this register can be freely accessed outside VBlank and HBlank.

FF69 — BGPD (CGB Mode only): Background palette data

As each color is two bytes in size, you must read/write this register twice to access a whole color.

This is made much easier through the use of the address auto-increment: BGPI’s “address” field is automatically incremented (possibly wrapping around) after each write to this register, even if the write fails due to CRAM being inaccessible. Reads, however, never trigger auto-increment.

FF6A–FF6B — OBPI, OBPD (CGB Mode only): OBJ palette index, OBJ palette data

These registers function exactly like BGPI and BGPD respectively; the 64 bytes of OBJ palette memory are entirely separate from Background palette memory, but function the same.

Note that while 4 colors are stored per OBJ palette, color #0 is never used, as it’s always transparent. It’s thus fine to write garbage values, or even leave color #0 uninitialized.

RGB Translation by CGBs

When developing graphics on PCs, note that the RGB values will have different appearance on CGB displays as on VGA/HDMI monitors calibrated to sRGB color. Because the GBC is not lit, the highest intensity will produce light gray rather than white. The intensities are not linear; the values $10-$1F will all appear very bright, while medium and darker colors are ranged at $00-0F.

The CGB display’s pigments aren’t perfectly saturated. This means the colors mix quite oddly: increasing the intensity of only one R/G/B color will also influence the other two R/G/B colors. For example, a color setting of $03EF (Blue=$00, Green=$1F, Red=$0F) will appear as Neon Green on VGA displays, but on the CGB it’ll produce a decently washed out Yellow. See the image above.

RGB Translation by GBAs

Even though GBA is described to be compatible to CGB games, most CGB games are completely unplayable on older GBAs because most colors are invisible (black). Of course, colors such like Black and White will appear the same on both CGB and GBA, but medium intensities are arranged completely different. Intensities in range $00–07 are invisible/black (unless eventually under best sunlight circumstances, and when gazing at the screen under obscure viewing angles), unfortunately, these intensities are regularly used by most existing CGB games for medium and darker colors.

This problem with low brightness levels does not affect later GBA SP units and Game Boy Player. Thus ideally, the player should have control of this brightness correction.

1. The 16th bit, bit 15, is ignored by the hardware. In discussion, that bit is generally clear (for example, the canonical pure white is 7FFF and not FFFF), but the hardware treats both identically: it’s fine to fill color RAM with $FF bytes to set it to all-white! ↩

2. 2 bytes/color × 4 colors/palette × 8 palettes = 64 bytes. ↩