Probably awkward to have a client standing there while you verify their bills.
The savvy escort will take the money, excuse herself to "freshen up" in the bathroom, then whip out the counterfeit pen and verify.
The problem is if it comes back dark brown. Does she then confront the client, and refuse the client and his funny money? What if it is a false positive, and the money is actually good? What if the client is acting in good faith and someone else passed him the bad money? What if the client is indeed the counterfeiter, and once the "jig is up", and his criminal enterprise is exposed, he gets nervous and suddenly becomes violent, takes out a pistol, and pistol whips or shoots the escort?