Game Boy Advance Buttons Controlling Color? Not Directly Possible.

You cannot use GBA buttons to directly change the system's color output or palette. Here's why:

Hardware Reality

• Fixed Palette per Game: Each GBA game stores its own 15-bit (32,768 color) palette data in ROM. The PPU (Picture Processing Unit) renders only what the game code tells it.
• Buttons = Input Only: Buttons (A,B,Start,Select,L,R,D-Pad) feed into KEYINPUT register (0x4000130). Game code reads this to trigger actions (jump, shoot, menu). No hardware path connects buttons → PPU palette registers.
• No System-Level Palette Control: Unlike GBC (which had system palette registers for DMG games), GBA has no BIOS function or hardware register for global color correction/filters.

What Is Possible (Via Game Code)

If you are the developer (writing homebrew / hacking ROM), you can program buttons to:

• Swap palette banks (e.g., Day/Night cycle, character color select).
• Apply color math effects (Blend modes: Alpha, Brightness, Contrast) via BLDCNT / BLDY / BLDALPHA registers.
• Trigger HDMA gradients (scanline effects) on VBlank.
• Remap VRAM palette memory (OBJ/BG) dynamically.

External Hardware Mods (Physical Color Change)

• Backlight / Frontlight Mods: Change screen illumination color (white, warm, RGB). Buttons can toggle if wired to mod controller (e.g., FunnyPlaying V2 IPS kit + button combo).
• FPGA / HDMI Mods (RetroDither, GBA Consolizer): Digital video out. Palette simulation (LCD ghosting, color correction) done in FPGA firmware. Buttons could trigger OSD menu to change filter.

Summary: Buttons → Game Logic → Palette Change (if programmed). Buttons ↛ Hardware Palette. For playing existing games: Impossible. For making games: Standard practice.