Result of inRange ?

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The hue value for yellow is something like 25 to 32 so you should use inRange() like

inRange(imageHSVMat, Scalar(25, 20, 20), Scalar(32, 255, 255), imageThreshMat);

And see the HSV-Color space chart here, using which you can chose correct HSV value.

And if you want know the HSV value of specific pixel use below code

char window_name[30] = "HSV Segmentation";
Mat src;

static void onMouse( int event, int x, int y, int f, void* ){
 Mat image=src.clone();
 Vec3b rgb=image.at<Vec3b>(y,x);
 int B=rgb.val[0];
 int G=rgb.val[1];
 int R=rgb.val[2];

  Mat HSV;
  Mat RGB=image(Rect(x,y,1,1));
  cvtColor(RGB, HSV,CV_BGR2HSV);

    Vec3b hsv=HSV.at<Vec3b>(0,0);
    int H=hsv.val[0];
    int S=hsv.val[1];
    int V=hsv.val[2];

    char name[30];
    sprintf(name,"B=%d",B);
    putText(image,name, Point(150,40) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false );

    sprintf(name,"G=%d",G);
    putText(image,name, Point(150,80) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false );

    sprintf(name,"R=%d",R);
    putText(image,name, Point(150,120) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false );

    sprintf(name,"H=%d",H);
    putText(image,name, Point(25,40) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false );

    sprintf(name,"S=%d",S);
    putText(image,name, Point(25,80) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false );

    sprintf(name,"V=%d",V);
    putText(image,name, Point(25,120) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false );

    sprintf(name,"X=%d",x);
    putText(image,name, Point(25,300) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,0,255), 2,8,false );

    sprintf(name,"Y=%d",y);
    putText(image,name, Point(25,340) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,0,255), 2,8,false );

 //imwrite("hsv.jpg",image);
 imshow( window_name, image );
}



int main(){
 src = imread("bgr.png",1);
 imshow(window_name,src);
 setMouseCallback( window_name, onMouse, 0 );
 waitKey();  
 }