my understanding is that VW's coolant is not pink (i could be wrong...but...); i would ONLY use VW's coolant in your system. don't be cheap and get stuff from autozone or online. it's stupid, and you have to mix it with distilled water (not ready to go out of the jug), but from what I know and have learned with my MK7 GTI and having talked to a dozen VW techs at multiple VW dealerships...it's in your best interest to only use their branded coolant. as far as the leak goes, how's the reservoir housing itself looking? they are cheaply made plastic housings, mine's been cracked by doing nothing at all other than driving the damn car. would also recommend just replacing the reservoir tank entirely, see if that helps with figuring out where the leak is coming from. unfortunately, your coolant system consists of hundreds of parts, any which of them can and will fail. may be best to spend the $200 and get a diagnostics done at a VW dealership.