The term "Tainted Coins" is often misinterpreted as a measure of provenance. That's understandable considering the traditional definition of the word "tainted" coupled with the reality that many Bitcoins actually have been used for what would be considered nefarious purposes by standard societal norms. In fact, it is a common occurrence to be holding or trading Bitcoins that have traveled dark roads. The only coins that have absolute clean history are freshly mined coins, and much of that enters the wild pretty quickly so does not stay "clean" for long. So while it is tempting to compare tainted coins to a $100 bill that has either been used to pay for narcotics or used to actually ingest narcotics by way of a rolled up straw, this is not what "tainted coins" are in Bitcoin vernacular.
In the jargon of the science of Bitcoin, tainted refers to something that's not only counter intuitive to the definition of the term "tainted", but it's actually a fairly confusing (and boring) concept. In this case of Bitcoins, taint is simply a measure of correlation between two (wallet) addresses. The variable expressed as taint is primarily used to express this wallet connection.
To make things even less intuitive, ALL Bitcoin addresses which have received a payment, ever, are "tainted". Taint in no way effects the value or fungibility of any coin since it is not a measure of any negative characteristic that would be associated with the traditional definition of "tainted". Actually, there is a serious problem if all your coins do not have some level of taint.
Sorry to chime in on this topic months after the OP. Stack is where misnomers come to perish, so I figured it is never too late.