As far as I can tell (they are so small, you really can't see), they start putting down a root as soon as they germinate. I think this is the case because they don't move when top watered, so they have to have something anchoring them to the peat moss.
I don't think that there is any color change when they start growing roots. When they first germinate, they have 2 "leaves" and these are the only leaves they will ever form without traps. Soon after those 2 leaves spread open, the itty bitty traps start growing. In my experience, the leaves are always green, but the traps can be anything between red and green.