My own experience in NZ - here we can get from a supplier called BNT, an American product called Redline Water-Wetter. Its is designed to mix happily with Blue/Green antifreeze, and has been tested to ensure nothing horrendous happens. Their website is http://www.redlineoil.com/Products.aspx?pcid=10
. I am running it in my 164, no problems. It can act independantly as an antifreeze/boil-point lowering agent on its own, but uniquely is formulated and tested to work in conjuction with both red and green glycol formulations, without chemically interacting with either, and protecting the engine bits. Interestingly, it has been exhaustively tested in NZ, and is used on power boats with their massive diesels, in Formula Fords, racing Porsche GT-class cars, Targa cars, and many other applications. I regularly check my coolant system, giving it the flush each season - so far its been really wonderful, with no loss of cooling - and actually improving quite a bit, temp drop overall - as well as no dessications, perishing on visual or leak testing, and no gunk in the header tank. So overall, I am well sold on it. Red Line happily provide independent test reports, which is a very good sign, as they are interested in protecting their customers cars first, rather than their reputations, something I liked a lot.
Please let me know what you think of this - and its use might explain purple coolant, as that could be a mix of Red Line + Blue Glycol.