Pink whitney residue?

My girlfriends and I love to drink Pink Whitney out of the bottle when we have our girls nights, but recently I’ve noticed that it leaves a strange black residue on our mouths that we can’t identify. There’s no stains on the cap or the lip of the bottle, it comes off easily, but we have no idea what it is or what causes it. It looks like this when we wipe our mouths with our hands. I’m not worried that it’s dangerous or anything like that, but it is a little annoying to have to run to wash our faces almost every time we take a sip. Any idea what it is/has anyone else seen it?