old gas/turpentine

Does it change color when it turns?
I have a fuel injected car that sat for a while, and I had it checked out- they told me the injectors are bad.
The thing is they are fairly new, and when I took the fuel filter off yesterday the line going to the front of the car had clean gas in it, the line going back to the tank was brown colored gas.
It was stalling at red lights but since I changed the filter it isn't anymore, but it is still running rough. I tried some fuel system/injector cleaner and it seemed to help a little.

So I am thinking I need to drain the tank, the problem is I cannot get it out easily, you have to take the exhaust off, the rear axle out, etc.

I was thinking I could disconnect the filter and power the pump up to get the brown gas out, but it has had a few tank fulls run through it already so just emptying it may not cure it.

Is there anything I can run through the tank to clean it out?