Embed gray image to color image

I want to embed grayscale image to color image using DCT watermarking technique. this is my source code but I failed to insert the gray image. how to fix it?

   %function of jpeg_scan
    function out=jpeg_scan(N,M);
    scan_order = zeros(N,M);
    scan_order(1,1) = 1;
    diag_down = 1; x = 1; y = 2;
    for k = 2:N*M,
    scan_order(x,y) = k;
    if diag_down==1, y=y-1;x=x+1; end;
    if diag_down==0, y=y+1;x=x-1; end;
    if y>N, y=N; x=x+2; diag_down=1; end;
    if x>M, x=M; y=y+2; diag_down=0; end;
    if y<1, y=1; diag_down=0; end;
    if x<1, x=1; diag_down=1; end;
    end;
    out=scan_order;  

%host image
  host=imread('Lena512.bmp');
  yCbCr = rgb2ycbcr(host);
  Y = double(yCbCr(:,:,1));
  [xm,xn] = size(Y);
  scan_order = jpeg_scan(xm,xn);

% call watermark image (grayscale image)
 watermark = imread('logo.bmp'); 
 Wm = ?????

    %transform Y component of host image to DCT
    Y_dct = dct2(Y);

    temp = Y_dct(:);
    zz_dct_koeff = temp;
    zz_dct_koeff(scan_order(:)) = temp;

    V = zz_dct_koeff(L+1:L+n);
    V = V + a*abs(V).*Wm;

    zz_dct_koeff(L+1:L+n) = V;
    temp = zz_dct_koeff(scan_order(:));
    Yx_dct = reshape(temp,xm,xn);
    Yx = uint8(idct2(Yx_dct));
    yCbCr(:,:,1) = Yx;
    wtm = ycbcr2rgb(yCbCr);

    imwrite(wtm,'watermarked_image.bmp','bmp')
  figure(1);imshow(host);title('HostImage')
  figure(2);imshow(wtm);title('WatermarkedImage')

Answer 1

[Faradisa]

please help me