BROWN I think, thick brown/yellow is from the dynamo to control box. Thin brown/yellow goes to ign warning lamp. It's been about four years since I metaphorically chucked control box and dynamo in the skip, and done the sensible thing and fitted an alternator, so I'm relying on memory and some guesswork. Some photos might help.
On the solenoid picture below, in which the white/blue wire is towards the front of the car:
+ The big black wire is going to the starter motor, it is on one terminal of the solenoid all by itself.
+ The thin white/blue goes to the coil live side, it provides the bypass full 12v when cranking the engine.
+ The thin white/red wire is live from the ignition switch, when key is held against the spring to operate starter.
+ The big red wire is from the battery. Under that, on the same post are two other thickish brown wires, also with ring type terminals, one is a brown wire supplying the fusebox and everything else on the car (I strongly recommend fitting a fusible link in this wire, sometime), the other brown wire under the red wire from the battery is the brown wire, carrying charging current from either of the two control box 'B' terminals)
On the dreaded control box, left to right:
+ The little brown wire is an earth for the control box itself, it goes into the loom and back out again, in this picture its other end has black paint on it, but it's the same wire, with a ring terminal under one of the bolts that secure the control box onto the inner wing.
+ Next (L-to-R) is a pair of brown/yellow wires, the thick one goes to the dynamo output terminal, the thinner one goes up to the instruments for the IGN warning lamp.
+ Next is brown/green striped, it goes from dynamo to control box 'F' (Field) terminal.
+ Next is the big brown wire, the other end of which you see in the solenoid photo above (it's splashed with green paint)
+ Last the black-ish wire is the other end of the little brown earth wire, mentioned above.
Hope this helps, I'm aware too you might be positive earth, in which case all bets are off!