I'm curious about this too and just asked my son's doctor about this over the phone the other day. I'm still waiting on a response. For him, color change is triggered by either cold air or if he is using his walker and dangling his foot (like when we go to the pool and he can't walk bare foot yet to get from locker room to water.) They said keeping it warm helps. I've noticed as soon as he puts it down and starts moving it again, it goes back to normal. So it seems like a circulation problem? I don't really understand it. What is LDN?