I’ve had my thermarest ridgerest for about 15 years and it’s been pretty well abused over my years in boy scouts. I found a good deal on a Thermarest Trail Pro for $35. The only downside is I mistakenly thought it was olive when it’s actually lime green. I’m thinking of taking some spray paint to it but I’m concerned it’s not going to stick and be a mess. It’s made of 50D/ 75D rip polyester according to Thermarest.
Dupli-Color makes a product called Vinyl&Fabric coating. It comes in a spray can. You can get it at most auto parts stores that sell car paints. That would probably be your best bet. It’s for dashboards, door panels, and seats. I don’t know how it would hold up being rolled and unrolled. I’ve used it on vinyl seats and it’s worked well.
Because you have to roll it up with the valve “up” or it will suck debris, and it has to be rolled towards the retainer straps, which means that always the same small area is exposed when it is rolled, and not the entire pad. So you really only need paint a small portion of the pad, and dust the end with paint when rolled.
Put it inside of your bivy, roll the bivy with sleeping bag and pad in it. You can either stuff all of this in an appropriate sized stuff sack, or put it in your kit as is. The thermarest isn’t really exposed this way, or paint, your choice. Play around with it, see what works for you.