dst = src.clone();
    cv::Mat Roi(dst, cv::Rect(x, y, cut_out_len, cut_out_len));
    cv::RNG rnger(cv::getTickCount());
    rnger.fill(Roi, cv::RNG::UNIFORM, cv::Scalar::all(0), cv::Scalar::all(256));

上述代码是将Roi区域进行随机的颜色填充

//创建一幅320×240的图像
Mat img(Size(320,240), CV_8UC3);
//选择一块ROI
Mat roi(img, Rect(10, 10, 100, 100));
//将ROI的颜色填充为绿色，原始的img图像将会改变。
roi=Scalar(0,255,0);

cv::Mat image = cv::Mat::zeros(512, 512, CV_8UC3);  
image.setTo(cv::Scalar(100, 0, 0));  
cv::imshow("original", image);  
cv::Mat roi = cv::imread("E:\\Images\\Hepburn.png", cv::IMREAD_COLOR);  
cv::imshow("roi", roi);  
cv::Rect roi_rect = cv::Rect(128, 128, roi.cols, roi.rows);  
roi.copyTo(image(roi_rect));  
cv::imshow("result", image);  
cv::waitKey(0);    
return; 

另外生成一定范围的随机数：https://www.cnblogs.com/vathena/articles/4356646.html

srand((unsigned)time(null));
(low,up) #define Random (rand()%(up-low+1)) + low - 1
[low,up) #define Random (rand()%(up-low)) + low
(low,up] #define Random (rand()%(up-low))+ low + 1
[low,up] #define Random (rand()%(up-low+1)) + low