[Toadling]

My impression has been that in GTD there is always a single next action, even if the project is parallel. Maybe think of it as the next, most likely or most important action, even though any number of actions could be performed instead or at the same time. I guess that's why OF shows the top-most action in purple in a parallel project.

I think the main difference between a sequential and parallel project in OF is that the remaining actions in a parallel project are not blocked (i.e. they're displayed in black rather than grayed-out text).