changing the bgr color values of a picture on opencv

Question

My function below should change the color values of the original picture and then once the process is done it should create another picture with the changed colors.

void change(const cv::Mat &first_image, cv::Mat &new_image, const std::vector<std::vector<int> > &colors)
{
    for (int i = 0; i < first_image.cols; i++) {
    for (int j = 0; j < first_image.rows; j++) {

 int blue = first_image.at<cv::Vec3b>(i,j)[0];
 new_image.at<cv::Vec3b>(i,j)[2]= colors [2][blue];

 int green = first_image.at<cv::Vec3b>(i,j)[1];

 new_image.at<cv::Vec3b>(i,j)[1]= colors [1][green];

 int red = first_image.at<cv::Vec3b>(i,j)[2];

 new_image.at<cv::Vec3b>(i,j)[0]= colors [0][red];
}
}
}

when I use the command make and run the whole code I get a segmentation fault. I use this command to run the code

../../build/mv/task/mvtask1d pic.json

Could someone tell me what I could be doing wrong?

Thank you in advance