The circuitry does exactly what you described, it detects the bit pattern for that specific color that would be "dark yellow", and when the bit pattern matches, an extra transistor is turned on to reduce green intensity.
In general, the digital R/G/B signals are used to add voltage to respective R/G/B analog signal, and the digital intensity signal adds to all three RGB channels.
More specifically, the digital RGBI input circuitry uses 74S05 inverting digital logic buffers with open-collector output to receive and buffer the four digital color signals. Because the buffers are inverting, the signals are inverted twice to end up back with a non-inverted signal that can be used to drive resistor networks, in order to convert the digital signals to analog levels for the transistors that drive the analog RGB signal levels.
So, because you have all the buffered digital RGBI signals, and their inverted complements available through inverters, and your inverters have open-collector outputs, you can combine the inverter output signals to make a wired-AND circuit.
The wired-AND circuit uses the non-inverted and inverted RGBI signals in such a way, that only one combination of them allows the output of "dark yellow" bit pattern detecting inverter to go high and modify green to darker, otherwise, the output stays low and green is not modified.
The logic is as follows:
B bit is high, inverted B bit stays low -> output is low
I bit is high, inverted I bit stays low -> output is low
G bit is low, buffered G bit stays low -> output is low
R bit is low, buffered R bit stays low -> output is low
So the only combination that allows the output to become is high, is low on blue and intensity, and high on red and green, RGBI = 1100, which would be dark yellow.
The output of the wired-AND circuit turns on a separate transistor to pull current away from the green analog line via a 560 ohm resistor, as otherwise it would get full current via a 150 ohm resistor, while green analog line would be OFF when pulled low via 220 ohm resistor.