That is what we do. We tell them that we have them available so they won't use opur good towels. I also have dark colored washcloth's in a little basket in the bathroom by the sink with a sign to please use them to remove make-up...never had any problems
You could always stock up on a few sets of all black towels and when you hear that trademark roar of a Harley pulling up, go throw them in the room they are staying in. LOL
Better yet, I'll bet Harley actually has a logo emblazoned black towel line available for the diehards.
We just had a couple traveling in their '65 Corvette and I kept waiting to see him grab one of the towels to clean his "baby", but lo and behold, he'd pop the trunk a few times a day and pull out this chamois duster, cleaning mitt, etc.
Three weeks on the road, barely an inch of space to spare for luggage, but an entire array of dusting and cleaning tools for the car. A guy just has to have his priorities right.
No wonder the wife asked if we had bathrobes, blow dryers and makeup removal stuff when she booked. No room in the car for them.
I thought about going the logo towel route but thought that might just be too tempting of an item to use and shove in the saddlebag - not to suggest that bikers would even think of such a dastardly deed - but just thinking...