[Dave Moll]

Of course, the giveaway on a monochrome set that a colour transmission was being received was the presence of chroma dots.

This has been exploited to recreate the colour from monochrome recordings of at least one colour transmission where the chrominance wasn't suppressed on the recording. Famously, this was done with an episode of Dad's Army where only mono recordings of the original colour programme existed, though a fair amount of processing was needed.