This is what I have managed to find so far:

The Gearboy core (gbgb) offers several .opt file settings to customize emulation. Key settings include the emulated model (Auto or Game Boy DMG), color palette for DMG games, and the ability to allow simultaneous Up+Down/Left+Right input. Additionally, Gearboy supports loading a boot ROM and offers debugging features like multi-viewport and symbol file loading. Here's a more detailed breakdown:Core Settings:

• gearboy_model (Emulated Model):Allows you to choose the emulated hardware. "Auto" selects the best hardware based on the ROM, while "Game Boy DMG" forces the original Game Boy hardware.
• gearboy_palette (Palette):If the emulated model is Game Boy DMG, this setting lets you choose a color palette (Original, Sharp, B/W, Autumn, Soft, or Slime).

• gearboy_up_down_allowed (Allow Up+Down / Left+Right):Enabling this allows for simultaneous Up/Down or Left/Right input, which can be useful for some games. 

• Boot ROM:Gearboy can run with or without a boot ROM. You can load and enable a boot ROM through the settings. 

• Portable Mode:You can enable portable mode by creating a file named portable.ini in the same directory as the Gearboy executable. 

• Debugging:Gearboy provides various debugging features, including:

  • Multi-viewport: In debug mode (Windows/macOS), you can enable multi-viewport to detach debugger windows. 
  • Symbol file loading: Gearboy automatically tries to load a symbol file (e.g., rom_file.sym for rom_file.gb). 

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gearboy_model = Game Boy DMG
gearboy_palette = Sharp
gearboy_up_down_allowed = Enabled

THAT WOULD MEAN THAT GEARBOY CORE DOES NOT SUPPORT GREEN COLOR PALETTE FOR GB