Look at any VHS capture and the luma is soft but sane, while the colour is a smeary mess that bleeds sideways and wobbles. Those are not the same problem being worse in one channel. They are two completely different recording systems sharing one tape.

The problem the VHS engineers had is that a helical scan head cannot record a 3.58 MHz colour subcarrier and 3 MHz of luma in the same FM signal without the two intermodulating into mud. So they did not try. They split them.

Luma gets FM modulated onto a carrier around 3.4 to 4.4 MHz and recorded that way, which is why VHS luma survives reasonably well. FM is robust against the amplitude wobble you get from a tape flapping past a spinning head.

Colour got demoted. The chroma subcarrier is heterodyned down to 629 kHz on NTSC decks, 627 kHz on PAL, and recorded underneath the luma FM as a plain amplitude modulated signal. That is the "colour under" system, and the name is literal. It sits under the luma in frequency.

Two consequences follow immediately.

The colour bandwidth collapses. You get roughly 400 kHz of chroma instead of the 1.3 MHz the broadcast signal carried, so colour detail smears horizontally over several times the width of a luma detail. That is the sideways bleed.

And the colour becomes fragile in a way the luma is not. AM has none of FM's immunity to amplitude variation, so every bit of tape dropout, head clog and tension wobble lands directly on the chroma. The luma shrugs it off and the colour does not.

There is a nice tell for the 629 kHz figure. It is exactly 40 times the NTSC line rate. Locking the colour under carrier to a multiple of the line frequency makes its interference pattern stationary rather than crawling, which is the same reason NTSC picked its own subcarrier the way it did.

Video is one clip clean, then through the tape path, with the chroma bandwidth dropping and the colour starting to wobble independently of the picture.

Why VHS colour looks worse than VHS brightness, and why that was deliberate