Leafy Spurge Hawk Moth caterpillar

I was so surprised when I saw that one of the three Leafy Spurge Hawk Moth caterpillars that I came across yesterday was a different colour. To show my ignorance, are caterpillars either male or female? I'll have to do some research : ). By the way, the end with the "horn" and two white eyes (you can only see one in the photo) is the tail end.