Read Video and subtract color

Hello,

I can open a video and convert to gray scale. But how can i subtract a color like red from grayscale? diff = imsubtract(vid(:,:,1),rpg2gray(vid)) didn't work.

I get the Error:

Error using rgb2gray>parse_inputs (line 77)
MAP must be a m x 3 array.
Error in rgb2gray (line 52)
[X, threeD] = parse_inputs(X);
Error in VideoTracking_Color_Version2 (line 52)
    sub_imgObj = imsubtract(imgObj(:,:,1), rgb2gray(imgObj(k)));

And this is a part of the code: hThresholds = [0.799, 0.990]; sThresholds = [0.480, 0.999]; vThresholds = [66, 202];

for k = 1:nFrames  
  imgObj=read(videoObject,k);
        sub_imgObj = imsubtract(imgObj(:,:,1), rgb2gray(imgObj(k)));
  ...