The question about how you can tell which one has what colo(u)r feet is very interesting. But it can be solved with just one question. (It assumes, BTW, that each one knows the colour of the other's feet - only you don't know.)

What is the one question about the colo(u)r of their feet that will inevitably reveal which is the truthful and which the lying man? (IOW, you may not question them about their own or the other persons truthfulness.)

PM the answers to me, so that premature publication on the board doesn't spoil other peoples fun.
I'll publish the answer and the winners on Thursday, 10 am BST.
Special prize if anyone comes up with a non-standard answer that works!