compareHist threshold

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Hi at all!!! I'm doing a comparison between two different images but I have a problem to set the threshold for three color: blue, green and red. Could anyone tell me how I set these thresholdes? Below I post my program and I am attaching the two different images. The scope of my program is detect object or people doing a comparison with background image and show parts that are different in red.

#include <iostream>
#include <stdio.h>
#include <math.h>
#include <opencv2\opencv.hpp>
#include "opencv\highgui.h"
#include "opencv\cv.h"
using namespace cv;
using namespace std;
void add_piece_of_frame(const Mat&, Mat&, int, int);
void add_piece_of_frame_red(const Mat&, Mat&, int, int);
int const dim = 20;
int main(int argc, char** argv)
{
    int const block = 80;
    //read image
    Mat background = imread("Scene1.jpg",CV_LOAD_IMAGE_COLOR);
    //check read image
    if(! background.data )                              
    {
        cout <<  "Could not open or find the image" << std::endl ;
        return -1;
    }
    //shows reference image
    imshow("background di riferimento", background);
    waitKey(1);
    //vectors for background histograms
    Mat hB_b[block]; 
    Mat hB_g[block];
    Mat hB_r[block]; 
    //vectors for image histograms 
    Mat hC_b[block];  
    Mat hC_g[block];   
    Mat hC_r[block]; 
    //**************variables for histograms*********************************************************************************
    //number of histogram bins
    int histSize = 256;
    //range per (b,g,r)
    float range[] = { 0, 256 } ;
    const float* histRange = { range };
    bool uniform = true;
    bool accumulate = false;
//**********************************************************************************************************************************************
    int count = 0; //counter variable for background cycle
    int count_c = 0; //counter variable for current cycle
    int nFrame = 0; //counter variable for numerated frames
    for(int a=0; a<320; a+=dim)
    {
        for(int b=40; b<140; b+=dim)
        {
            //block 
            Mat SUPPORT (background, Rect(a,b,dim,dim));
            //split block to obtain color elements
            vector<Mat> bgr_planes;
            split( SUPPORT, bgr_planes ); 
            //calculate histograms for each color
            calcHist( &bgr_planes[0], 1, 0, Mat(), hB_b[count], 1, &histSize, &histRange, uniform, accumulate );//blu
            calcHist( &bgr_planes[1], 1, 0, Mat(), hB_g[count], 1, &histSize, &histRange, uniform, accumulate );//green
            calcHist( &bgr_planes[2], 1, 0, Mat(), hB_r[count], 1, &histSize, &histRange, uniform, accumulate );//red
            count++;
        }
    }
    //VideoCapture cap("tagliato.avi");
    Mat current_frame = imread("Scene1_2.jpg",CV_LOAD_IMAGE_COLOR);
    //while(1)
    //{
        //Mat current_frame;
        //bool img = cap.read(current_frame); 
        //if(!img) 
            //break;
        //---------Mats' vector for recostruction image  (save parts of original image) ---------------------
        Mat recostruct[block];
        //matrix for modified output
        //Mat output = cv::Mat::zeros(240, 320, CV_8UC3);
        Mat output_chi_sq = current_frame.clone();
        Mat output_correl = current_frame.clone();
        Mat output_intersect = current_frame.clone();
        //vettori per il salvataggio dei punti iniziali di ricostruzione
        int aa[block];
        int bb[block];
        for(int a=0; a<320; a+=dim)
        {
            for(int b=40; b<140; b+=dim)
            {
                Mat SUPPORT_2 (current_frame, Rect(a,b,dim,dim));
                //save block to recostruction
                recostruct[count_c] = SUPPORT_2.clone();
                //save block's points
                aa[count_c] = a;
                bb[count_c] = b;
                //split current block
                vector<Mat> bgr_planes_c;
                split( SUPPORT_2, bgr_planes_c );
                //calculate histograms for each color
                calcHist( &bgr_planes_c[0], 1, 0, Mat(), hC_b[count_c], 1, &histSize, &histRange, uniform, accumulate );//blu
                calcHist( &bgr_planes_c[1], 1, 0, Mat(), hC_g[count_c], 1, &histSize, &histRange, uniform, accumulate );//gren
                calcHist( &bgr_planes_c[2], 1, 0, Mat(), hC_r[count_c], 1, &histSize, &histRange, uniform, accumulate );//red ...