In the phase splitters the chroma signal becomes either positive or negative, which if you recall the video from earlier make it so the signal can cover all portions or the diagram (recall him talking about +I/-Q and so forth). Then the completely demodulated chroma signal in both positive and negative forms are combined with brightness information from the video amplifier in the correct proportion by the matrix resistors, and applied to the amplifiers for each primary color.
Later receivers greatly simplified this concept by doing away with the matrix and color amp (called output) tubes by combining the signals at the CRT itself, which if you recall I said earlier turns the CRT itself into a sort of mixer. A great example of this is the simplified color receiver RCA CTC-4, which didn't even have color amp tubes! Note that there are still 'demod' tubes which use high level demod to drive the CRT directly, but I suspect that if the signals were strong enough they could have been directly applied to the CRT doing away with even more circuits. A Muntz'd design...
Genius, no?