c++ and opencv get and set pixel color to Mat

I'm trying to set a new color value to some pixel into a cv::Mat image my code is below:

    Mat image = img;
    for(int y=0;y<img.rows;y++)
    {
        for(int x=0;x<img.cols;x++)
        {
        Vec3b color = image.at<Vec3b>(Point(x,y));
        if(color[0] > 150 && color[1] > 150 && color[2] > 150)
        {
        	color[0] = 0;
        	color[1] = 0;
        	color[2] = 0;
        	cout << "Pixel >200 :" << x << "," << y << endl;
        }
        else
        {
        	color.val[0] = 255;
        	color.val[1] = 255;
        	color.val[2] = 255;
        }
    }
    imwrite("../images/imgopti"+to_string(i)+".tiff",image);

It seems to get the good pixel in output (with cout) however in the output image (with imwrite) the pixel concerned aren't modified. I have already tried using color.val[0].. I still can't figure out why the pixel colors in the output image dont change. thanks

You did everything except copying the new pixel value back to the image.

This line takes a copy of the pixel into a local variable:

Vec3b color = image.at<Vec3b>(Point(x,y));

So, after changing color as you require, just set it back like this:

image.at<Vec3b>(Point(x,y)) = color;

So, in full, something like this:

Mat image = img;
for(int y=0;y<img.rows;y++)
{
    for(int x=0;x<img.cols;x++)
    {
        // get pixel
        Vec3b & color = image.at<Vec3b>(y,x);

        // ... do something to the color ....
        color[0] = 13;
        color[1] = 13;
        color[2] = 13;

        // set pixel
        //image.at<Vec3b>(Point(x,y)) = color;
        //if you copy value
    }
}

just use a reference:

Vec3b & color = image.at<Vec3b>(y,x);
color[2] = 13;

I would not use .at for performance reasons.

Define a struct:

//#pragma pack(push, 2) //not useful (see comments below)
struct BGR {
   	uchar blue;
    uchar green;
   	uchar red;	};

And then use it like this on your cv::Mat image:

BGR& bgr = image.ptr<BGR>(y)[x];

image.ptr(y) gives you a pointer to the scanline y. And iterate through the pixels with loops of x and y