Questions about 1983 V20 fuel tank

[bilgsmf]

There was a total of 21 gal in my tank (60 gal capacity) that I syphoned out with a DC portable pump from Amazon. The gas was a green color, not sure what that color means? i took my gas to the recycling center. My question now is how do I clean the inner tank of any residue left by gas sitting so long in my tank. I was thinking of just putting in maybe 5 gal of gas and then just polish it for several hours to get all of the leftover gunk that my syphoning did not get. Ideally if I had the tank out I could wash it out and make sure that it was clean, but tank wont be removed. Any advise would be appreciated.