I just tried this myself, it works a treat.
The other thing I have found also works, is, empty the tank, remove brackets, petcap and petcock. Soak the whole tank in the laundry tub for 24 hrs (Napisan). Brush off any loose dirt spots, whilst in the liquor. Drain tub, rinse off any residue. Repeat once (or more if necessary).
If you want to put a couple of tablespoons of liquid bleach for good measure, then that is up to your disgretion.
This worked for me for the last 2 years.
The staining and discolouring comes back.
Soon I will look at a replica tank to match the rest of the plastics on the bike (NOS).