If its a DT250 or similar the headlight only run with the motor running, magnetó powered lights. If you're in the US the tail light might be battery powered. So battery-- "i'm guessing a little"-- will have power in the red going to the switch, then brown should be live out of the switch when switch is on. Brown DC will power things like indicators, brake light, horn, neutral light & possibly tail light but that might be another color but still dc in US. Magneto will power headlight & instrument illuminating bulbs. Then Blinkers ie indicators will be dark green & dark brown left & right--not sure which is left but can't go very wrong--dark brown left i think. I'll leave it there now i've confused everyone including myself lol. Welcome if you're new, or welcome back.
I forgot "no spark''. Good old magneto might just need to give the points a scratch to wake it up.

The lighter brown is "key switched power".

The darker ("chocolate") brown wires are the output wires from handlebar switch to the left turn signals.

In your picture it looks like someone may have connected dark brown to lighter brown which is not correct.

Good spot, Mark!

Looks like a vacant port in the dark brown triple female right next to it.

Ok I made some progress, thanks guys. I removed rhe main switch, polished a connection, and changed out the plug. I am now getting voltage through the switch and, when the key is turned, to the two sets of light brown female plugs in the headlight area. But what do I plug into thise? The diagram is no clear. The blue and yellow circuits still seem un-powered. Does there need to be some cross-pollination between the colors here? 

Which model do you have and are you in the US, or somewhere else?

Front brake light switch will use one (lighter) brown female. Is that a green/yellow female that I see taped up? That will be for the other wire from the front brake light switch. 

I am in the US and have a 75 DT250.

yeah I tape the G/Y off because there currently is no front brake switch haha. I sources one off ebay, but it’s not here yet.

Also, it still just seems like the blue and yellow circuits are un-powered up front, although the taillight and rear brake light are both working. That seems right, but neither the indicators or gauge lights are working. I am tempted to troubleshoot the lack of spark first, since I know some of these lights won’t work until the engine is running. 

Good plan?

For the spark, try giving the faces of the points a clean. I'm lucky enough to have a proper points file, but others use emery or even fine sandpaper. 

Good call, I just installed brand new ones though haha. If I just rotate the flywheel by hand, should the engine circuits (headlight etc) power on?

No.  Like to like for the most part.  Sometimes you have to use "common sense" as well as there are times when Yamaha left a stripe off a wire or something.  On the later bikes like yours they often used plastic connectors for some of the wires that could be mixed up so it's usually hard to screw up. 

Yellow/white (might not have the white stripe sometimes) coming into headlight bucket is an AC circuit powered only when engine is running and powers the handlebar headlight switch on your model.  Another yellow in headlight bucket is high beam out of headlight switch so headlight needs to be on high beam to get power there. 

There is a blue out of headlight switch that runs taillight off battery brown with key on and light switch on.  There are also blue wires that are powered off engine (yellow/white) that power gauge illumination lights when engine is running and headlight switch on.

I don't remember if those two different blues have stripes or different shades of blue on 75 DT250.  Or they might be in plastic plugs so they can't be connected wrong. 

Example of "common sense":  If there are two blues in headlight bucket coming from headlight switch you can't figure out, one coming out of headlight switch will have battery power with key on and headlight switch on.  That blue connects to harness going to back of bike to taillight.   The other blue coming out of headlight switch goes to blues for the tach and speedo gauge illumination lights right in the headlight bucket, it's powered off the engine and through the headlight switch. 

Looking at wiring diagram and getting a sense of what comes from where and goes to what is very helpful. 

I can see in your pic there is a brown/white female and brown/white male that are disconnected.  Those are the only brown/whites and need to connect if you have turn signals.  A quick look at diagram shows brown/white comes from turn signal flasher to power the signals and goes to brown/white into turn signal switch on handlebar.  If turn signal switch is pushed left, the switch connects the dark (chocolate) brown out of the handlebar switch to the brown/white from the flasher.   If the turn signal switch is pushed right, the switch connects the the dark green out of the handlebar switch to the power from the flasher brown/white. 

All the circuits can be figured out in a similar way. 

 

A bunch of posts came in while I was writing. You need to get engine running for headlight, gauge illumination, and high beam warning. Everything else runs off battery.