For the most part:
On the "small" taillights, the GREEN is Ground (like the rest of the bike) and the GREEN/STRIPE (stripe color varied) is brake. The BROWN is the running taillight.
On K2 and early K3, the GREEN/STRIPE changed the stripe color, then became [mostly] BROWN/WHITE during the K3 for BRAKE.
On the K4 and later, the BROWN/WHITE remained the brake for a long time, but sometimes the stripe was dots instead.
Caveat: when Honda started their second 750 factory (near the middle of K1 production), these particular wire colors were not real consistent. This happened again around the K5 time. The GREEN and BROWN always were the same, but the third wire for the brakelight changed several times for short periods, even not matching the harness connectors under the seat! I have seen the following colors for that brakelight wire, just on the taillight itself:
GREEN/WHITE
GREEN/BROWN
BROWN/WHITE
BROWN/GREEN
BROWN with WHITE DOTS
Since this taillight also fits the 450, 500T, 500, 550, and even for a while the 360T, it was probably a build-what-we-have-today situation for Honda in those days.
The wiring harnesses mostly used GREEN/STRIPE (often YELLOW stripe, sometimes WHITE) or else BROWN/STRIPE (mostly WHITE stripe, sometimes RED) to carry the brakelight circuit to the two brake switches.
