Tool/Function in OpenCV similar to imtool() in MATLAB

edit

using the code from your link, you are already very close.

all it needs now is access to your image in the mouse handler:

// in main:
// Read image from file 
Mat img = imread("MyPic.JPG");
//set the callback function for any mouse event
setMouseCallback("My Window", CallBackFunc, &img); // pass the img address to callback


// callback:
void CallBackFunc(int event, int x, int y, int flags, void* userdata)
{
    Mat img = *((Mat*)userdata); // 1st cast , then deref

    if  ( event == EVENT_LBUTTONDOWN )
    {
        Vec3b pixel = img.at<Vec3b>(y,x); // y,x here !
        cout << "Left button clicked - position (" << x << ", " << y << ")" << endl;
        cout << "color (" << int(pixel[0])  << ", " << int(pixel[1])   << ", " << int(pixel[2])  << ")" << endl;
    }
}