Finally had a chance to dig the old books out of the ba***t.
The reasons given for the change in H freq from 15750 to 15734.264 and V
freq from 60 to 59.94, and the selection of 3.579545 MHz as the color
subcarrier frequency are two fold:
1) To interleave the color information into the spectrum between the H-freq
multiples of the luminance information (color subcarrier frequency is an odd
multiple (455th) of one-half the H scan frequency)
2) minimizing the herringbone effect caused by a beat (approximately 920
kHz) between the color subcarrier and the 4.5 MHz sound IF (this is
accomplished by making the subcarrier frequency such that the BEAT frequency
between the 4.5 IF and the subcarrier is also an odd multiple (117th) of
one-half the H scan frequency).
By choosing these frequencies, any chrominance information that shows up in
the luminance channel is of opposite polarities on successive scans, and is
effectively cancelled out by persistence of vision.
Likewise, any beat-frequency interference at the 920.455 kHz point (A-B mix
of the 4.5 MHz IF and the 3.579545 MHz subcarrier frequency) is also
cancelled by persistence of vision.
These changes did not cause any problems for B&W sets, as the change
amounted to 1/10 of one percent of the 'old' standards.