I thought this looked like an interesting experiment to try. So I went and ransacked my cupboards for the darkest color food item I could find. In this case "black rice".
I crushed it down a little, mixed it with rubbing alcohol in a soup bowl, and spooned it over the foot of my paper towel tube.
This is what it looked like right after I did it.
This is 1 hour, 25 minutes later. Apparently black rice is actually dark red!