A greyscale image has no colour information, but you can still represent it as ycbcr if you wish - though it's not clear why you'd want to, since the chrominance planes will be constants. Anyway, this will convert, assuming you're starting from a 2D greyscale image:
rgbimage(:,:,1) = greyimage;
rgbimage(:,:,2) = greyimage;
rgbimage(:,:,3) = greyimage;
ycbcrimage = rgb2ycbcr(rgbimage);
You can also convert a colormap using the same function, if that's what you want to do.
