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
}
}