video watermarking with an image using dct

I am using dct2 to water mark a video using an image. After watermarking all the frames of the video i am trying to play back the watermarked image as a video. I am getting the following error-

Indexed movie data values must be legal colormap indices:

1.0 <= value <= length(colormap) for double-precision movie data, and

0 <= value <= length(colormap)-1 for uint8 movie data

Error in ==> rupali at 114

    M(re)=im2frame(wtrinage,colo);

The code used is--

re=1;

colo=colormap(gray);

for len=3:2:n

    watermark_image=strcat('WatermarkImage ',num2str(len),'.jpg');

    wtrinage=imread(watermark_image);

    M(re)=im2frame(wtrinage,colo);
    re=re+1;

end

movie(M)

I am not able to understand the error. Please help as soon as possible. Thanks