I used to do research and development at RCA Labs years ago on this problem. It is very hard to do it well. A reason that we get away with imperfect separation is that the portions of the luma image and of the chroma subcarrier (in terms of areas of the image on the CRT screen) track each other, and that our brains can pretty much ignore the crosstalk. Now if the two signals were unrelated, the crosstalk would be much more objectionable.

Hi, it is easy to remove the colour from the video signal. Look how I did it on my
UVB. Using the Y output does not solve the teletext problem and the problem
with the missing pedestals.

Kind regards
Darius