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you can replace

circle(srcImg, Point(j,i), 3, Scalar(0, 0, 128,128));

with

srcImg.at<Vec3b>(i, j) = srcImg.at<Vec3b>(i, j) - Vec3b(100, 100, 0);

but i prefer the way below

#include<opencv2/core.hpp>
#include<opencv2/imgproc.hpp>
#include<opencv2/highgui.hpp>

using namespace cv;

int main(int argc, char** argv)
{
    Mat srcImg = imread("image.png");
    Mat mask = imread("mask.png", IMREAD_GRAYSCALE) > 200;

    Mat red;
    cvtColor(mask, red, COLOR_GRAY2BGR);
    red = (red - Scalar(0, 0, 255)) / 2;
    srcImg = srcImg - red;

    imshow("image", srcImg);

    waitKey();
    return 0;
}