Hi Gruditch!
You got me examining the legs on the Marbled White pics I took yesterday - well, what else are Sunday mornings for?!
I noticed the same phenomenon amongst mine. In the close-up below, you can make out on the upper left leg a section of white on black. This makes me wonder whether Marbled Whites, when freshly emerged, have some white 'down' on their legs, which soon rubs off with wear and tear:
Either that or your anomalous Marbled White is just a bit kinky and likes wearing tights
Bryan