Louie the GUI is a tile based immediate GUI intended for microcontrollers.

Tilesets are 1bpp.

typedef struct louie_tileset_t louie_tileset_t;
struct louie_tileset_t {
    uint8_t tile_width;
    uint8_t tile_height;
    uint16_t reserved;
    uint16_t first_tile;
    uint16_t last_tile;
    uint8_t tile_data[];
};

Tilemaps are 16-bit. The 4 most significant bits are two color entries, which reference a device specific, 4-entry palette map.