Here are some more fun photos of Acronicta lepetita from Texas. Some of them turn orange during their final instar, some stay green, some even turn purple-ish. Luckily this species is quite sedentary, so it is relatively easy to photograph. This first shot is one of my favorites:
Many caterpillars are known to turn purple-ish, red-ish, or pink-ish just before pupation. However this orange color change happens while the caterpillar is still feeding, sometimes up to a week before pupation. It happens in Acronicta vinnula as well. I wonder why a caterpillar would want to be orange while still feeding on green vegetation?