problem in getting grayscale image from rgb image

Hi,

In this program when converting background image(BG) to grayscale, I'm getting a problem, but not with current image im. The update_background function is attached here please help me.

clc;
clear all
close all;

vid = mmreader('hall_monitor.mpg'); Nframe = vid.NumberOfFrames;

L = 5;
%%WEBCAM
delete(imaqfind)

% vid=videoinput('winvideo',1,'YUY2_320x240');

vid=videoinput('winvideo',1);
triggerconfig(vid,'manual');
set(vid,'FramesPerTrigger',1 );
set(vid,'TriggerRepeat', Inf);
% start(vid);
% vid = videoinput('winvideo',1);

%  View the default color space used for the data — The value of the ReturnedColorSpace property indicates the color space of the image data.
color_spec=vid.ReturnedColorSpace;

% Modify the color space used for the data — To change the color space of the returned image data, set the value of the ReturnedColorSpace property.
if  ~strcmp(color_spec,'rgb')
    set(vid,'ReturnedColorSpace','rgb');
end

start(vid)
pause
%%Detect background
trigger(vid);
im=getdata(vid,1); % Get the frame in im
[nr nc nm ] = size(im);
nf = L;
frames = zeros(nr,nc,nm,L);

for ii = 1:L 
%     ii;
    trigger(vid);
    im=getdata(vid,1);% Get the frame in im
%     im=rgb2hsv(im);
%     figure, imshow(im);
       G = fspecial('gaussian',[9 9],2);
%# Filter it
 im = imfilter(im,G,'same');
%     end
%      figure,imshow(im)
    frames(:,:,:,ii) = im;
end

% BG =updateBG( frames);
BG = update_background(frames);
BG= rgb2gray(BG);
% raj=BG(:,:,1);
% toc
% clear  frames
subplot(231)
imshow(uint8(BG))
title('BACKGROUND')
T=5;
  
%%Moving object Detection

for ii = 1:150
    tic
    trigger(vid);
    im=getdata(vid,1); % Get the frame in im
%     im=rgb2hsv(im);
      G = fspecial('gaussian',[9 9],2);
%# Filter it
im = imfilter(im,G,'same');
im= rgb2gray(im);
%     subplot(131)
    subplot(232)
     imshow(im)
    title(['Frame = ' num2str(ii)])
end