adding color to grey scale video

For NTSC, YCbCr is not trivial to colour as it is. It will usually be running with Y interleaved with alternating Cb and Cr. This is because the human eye only needs half of the colour information compared to luminance, so you get the following data stream:

Y0 Cb0 Y1 Cr0 Y2 Cb1 Y3 Cr1 etc.

This makes it very hard to colourise a 422 data stream. Its much easier to think in terms of RGB and then colour space convert it to 444 YCbCr then do the data rate conversion to make it 422 (you just take adjacent CB/Cr values and divide them by 2).

see this article on wikipedia:

http://en.wikipedia.org/wiki/ycbcr

Basically, its not simple to do.