Tamiya's Flat Base is, indeed, an additive to make gloss colors flat. I found this out the hard way when the Tamiya paints first came out.
I was about 12-14, somewhere in there, and I built a beautiful Monogram 48th P-47D in far east RAF colors (it was beautiful to a 12-year old, anyway!). I needed to tone down the decals and the gloss I had used under them. I was still brushing paint at this stage and I laid on a nice thick coat of Flat Base over the entire model.
Came back the next day and the whole thing had turned a rather inconsistent white, with some of the paint and markings barely showing thru! You live and you learn, but it gave me an idea which I have yet to try to this day. Perhaps someday in the near future I shall build a Bf 109 in temporary winter camo...
Fade to Black...