If you've looked at the TMS9929A datasheet, you may have noticed it already outputs Y, R-Y and B-Y signals. But if you tried to hook these up to a component television, you may have noticed the blues are too strong and the reds are too weak.
The reason is due to the way television detect the black-level of the signal, and the way the TMS9929A designers assumed their device would be used. This picture is from the datasheet. Figure 5-10 shows the Y channel's timing. The sync pulse is the first negative going pulse to the left. This synchronises the television to the incoming video signal. Immediately next to that is the 'back porch'. This is assumed to be at black level and the television samples this point for reference.
Figure 5-11 shows a pulse on the back porch for the R-Y and B-Y signals. This is intended to activate the colour burst on a PAL modulator. Unfortunately, if you connect this up to a component television, it will sample that point and treat it as the black level (or rather the 'no colour' level). Because it isn't black, there will be an offset error which causes the faulty colours.
The trick with my circuit is to suppress the colour burst pulse.