Analog video chrominance decoding - PAL/NTSC
You need to adjust the DDS generated subcarrier to match the actual colorburst reference signal. So they must be compared (frequency and phase) and based on the result the DDS phase increment is tuned so that the generated burst matches burst of the sampled video do match. How to actually do that, there might be many ways, and there are many video technology books that describe digital decoding of sampled composite video.