Next versus Available
 

If I told OmniFocus that I don't care whether or not I "Buy This" or "Buy That" first, why would "Buy This" be highlighted purple and show up in its own special "Next" filter?

I know it's a legacy kGTD but I don't get it. What does "Next" do for me?

Because even if Buy This isn't re prerequisite for Buy That, it's the action which will move the project forward, hence Next Action.

Read GTD.

Both actions are Available but only one can be Next in the "Buy these things project" (whatever you call the project). If this project is set to hold Singletons, then they will be (not yet implemented) blue instead of purple and they will all be Next Actions.

According to the ninjas the current behaviour is a compromise to try and accommodate some fairly disparate feature requests they've had about this. A workaround that Brian from Omni suggested to me is:

select a next action in Project view
go to format > copy style
select an available action in project view
format > paste style

Then all your available actions will appear the same as next ones. If you don't like purple you could do it the other way round I suppose. :)

Thanks pjb! That's a big part of my concern, and the differently-colored singletons is a better solution than I was hoping for.