"color information modulated on top of it at 4.58 mHz."

Two colour difference signals are used to phase modulate a 3.58 MHz subcarrier, not a 4.58 milli Hert subcarrier (please review SI units)

"If I understand correctly, "

You don't, you should have looked it up and copied and pasted, instead of the really bad explanation you posted.

"(Y) the instantaneous green signal level plus the sync signals; "

No, "Y" is the weighted sum of R, G and B, it is the luminence signal, and contians horizontal and vertical sync.

"(Pb) a signal that is the instantaneous blue signal level minus the instantaneous green level; and"

Pb is B-Y, which is equvalent to B - (R + G + B) = R + G (yellow)
Pr is R-Y, which is equivalent to R - (R + G + B) = G + B (cyan)

You need to get from thes baseband colour difference signals to a quadrature phase modulated subcarrier...a simple adaptor won't do this.