Why color reappears after Back (reverse) replay

Analog color TVs require an 3.58 MHz color burst signal to display a color picture (I remember seeing my first NBC color Peacock, probably in the mid 1960s). Which is analog, of course. http://en.wikipedia.org/wiki/Color_burst.
In MPEG, which is digital, the relevant information is represented by a binary number how I have not been able to determine.
The binary number has to be transformed into the anlog color burst signal by the IRD/Tivo. When you can reverse and replay the segment that previously was B/W and it is now color means the binary information was OK. Which means, for whatever reason, the IRD/Tivo on the first pass did not do a proper job.
When this has happened to me, the B/W picture displays no hint of color which probably means the color burst is missing. The color is always there and correct on "Back" to replay B/W segment.

Interestingly, B/W movies on TCM and Encore Westerns, the movie channels I watch the most (I'm get'n old ) frequently exhibit color artifacts which probably means the IRD/Tivo generates a color burst when it is not needed. (Just figured this out )
Conclusion: IRD/Tivo occasionally makes an annoying mistake!