Is the towel damp? I notice when the grass gets wet it leaves a green stain

I can't help, since I've never kept paper towel in my containers, so I've not seen stains like that before. It might be normal, or they might have very wet frass. Are the cats all seeming to behave normally?

I always find yellowish green to medium green liquid around the frass on the paper towels. The frass also is sometimes very green and wetter instead of the usual black dry ones. I have never found it to be indication of any problems. They only thing that is for sure bad is a comparatively large amount of very dark brown to black liquid which seems to happen when they are molting and diseased. Then I will usually find that cat dead later in more dark liquid. I have only had that happen a handful of times though.

They all seem fine today. I think I hit the panic button for nothing. However, I am like the old woman in the shoe--keeping my fingers crossed I can feed them all. Just found three more big ones outside on the plants I have been using for the inside ones.
Liz

I can't help you. And I don't know. I suggest asking Monarch butterfly web sites. Had that happen to me before, when I used to, in the past, raise them on the ground away from living Milkweed.

A very beautiful caterpillar.