The colours for trailing junction depends on the speed of the turnout compared to the line speed.
At the ARTC loop at Glenfield, the turnout appear to be high speed and the starting signal from the loop shows G/G.
At Gordon, the X40 turnout from the loop platform (P1) shows Y/Y as the line speed is 80.
At Chatswood, inconsistantly, the X80 crossover from P2 to the city shows Y/Y even though the line speed is probably 80km/h as well. Go figure.
There can also be a difference between a standing start from a deadend siding or platform and a through route from a branch line.
It sounds logical. If only it were consistently applied. As you said, some have upper greens and others upper yellow, irrespective of the speed reduction required.
The other thing is that with a trailing junction there is only one route option. Under route signalling, drivers are supposed to have route knowledge, i.e. firstly to know that the points are there and secondly to know their speed. If there is only one route option, the speed through the points is effectively the line speed for that short section, and it should
be well known by anyone on the footplate, just as they should be well aware of sharp curves that require hefty brake applications. Reliance on route knowledge can be taken too far and route signal indications can become ambiguous, e.g. Foxhall Junction 27 September 1967 and Concord West NSW, but they were overspeed derailments on facing points, not a convergence.
In fact, there are even some facing junctions or trailing junctions with trap points where I think an upper green would be more appropriate (ignoring the few "high speed" junctions in NSW). Again, where there is only one main route option, approach speeds are low or from a standstill, but it seems that since points are required reverse for the movement, it "must" have an upper yellow. There really is no need for the "whoa, slow down, points ahead!" cautionary effect of the upper yellow in such cases.
I suppose it might be cheaper to provide a Y/R+Y/R signal than G/R+G/Y/R/g, possibly with route indicators. Taking that further, it is probably also cheaper to simply hold the signal in rear at medium than resorting to the approach control method used in the UK. Having said that, I doubt the odd looking NSW single colourlight turnout signals combined with fixed reds found at converging junctions are cheaper than a standard three light signal head, which in many cases I'm sure could be substituted quite satisfactorily given the low speeds involved where many of them are used. Their four aspect capability would provide more information about the condition of the line ahead.