您没有在此处初始化G值

    data.b = 0;
    data.r = 0;
    data.b = 0;

还包括第一个结构开头的#pragma pack（push，1)和最后一个结构结尾的#pragma pack(pop)

#include <iostream>
#include <fstream>
#include <stdio.h>

using namespace std;

#define BYTES_PER_PIXEL 3
#define FILE_HEADER_SIZE 14
#define INFO_HEADER_SIZE 40

#pragma pack(push, 1)

struct BITMAPFILEHEADER {
    unsigned short bfType;
    unsigned int bfSize;
    unsigned short bfReserved1;
    unsigned short bfReserved2;
    unsigned int bfOffBits;
};
struct BITMAPINFOHEADER {
    unsigned int biSize;
    unsigned int biWidth;  
    unsigned int biHeight;
    unsigned short biPlanes;
    unsigned short biBitCount;  
    unsigned int biCompression;
    unsigned int biSizeImage;
    unsigned int biXPelsPerMeter;
    unsigned int biYPelsPerMeter;
    unsigned int biClrUsed;
    unsigned int biClrImportant;
};

struct RGB_data {
    unsigned char b;
    unsigned char g;
    unsigned char r;
};

#pragma pack(pop)

int main() {
    size_t height = 100;
    size_t width = 50;
    size_t size = width * height * 3;
    //------------------------------------------------------------------------------
    FILE *file = fopen("./sample.bmp", "wb");
    //------------------------------------------------------------------------------
    BITMAPFILEHEADER bmfh;
    BITMAPINFOHEADER bmih;
    //------------------------------------------------------------------------------
    RGB_data buffer[height][width];
    for (int y = 0; y < height; y++) {
        for (int x = 0; x < width; x++) {
            RGB_data data;
            data.b = 0;
            data.r = 0;
            data.b = 0;
            buffer[y][x] = data;
        }
    }
    //------------------------------------------------------------------------------
    bmfh.bfType = 0x4D42;
    bmfh.bfSize = size + FILE_HEADER_SIZE + INFO_HEADER_SIZE;
    bmfh.bfReserved1 = 0;
    bmfh.bfReserved2 = 0;
    bmfh.bfOffBits = bmfh.bfSize - size;
    //------------------------------------------------------------------------------
    bmih.biSize = 40;
    bmih.biWidth = width;
    bmih.biHeight = height;
    bmih.biPlanes = 1;
    bmih.biBitCount = 24;
    bmih.biCompression = 0;
    bmih.biSizeImage = size;
    bmih.biXPelsPerMeter = 0;
    bmih.biYPelsPerMeter = 0;
    bmih.biClrUsed = 0;
    bmih.biClrImportant = 0;
    //------------------------------------------------------------------------------
    for (int y = 0; y < height; y++) {
        for (int x = 0; x < width; x++) {
            buffer[y][x].b = 10; 
            buffer[y][x].r = 20;
            buffer[y][x].g = 30;  
        }
    }
    //------------------------------------------------------------------------------
    fwrite(&bmfh, 1, FILE_HEADER_SIZE, file);
    fwrite(&bmih, 1, INFO_HEADER_SIZE, file);
    fwrite(&buffer, 3, size, file);
    fclose(file);
    //------------------------------------------------------------------------------ 
    cout << "Image generated successfully!" << endl;
    return 0;
}