Well, my plants stuck out in the clay and growing happily for many years now are a mix of tan and green, but mostly green. We've had many frosts - I suspect you wouldn't have had any. I don't know, having never gardened out there, but my guess is they'd likely be evergreen there (since they're partially evergreen here). So, adding it all up, rightly or wrongly, yellow doesn't sound like a good color for them! It often means too much water or too little. Given that one plant is in a hypertufa trough, it's hard to imagine it having insufficient drainage and thus getting too much water despite your rainy fall (though I did notice that troughs are generally put up on blocks at the alpine gardens I've seen out there, presumably to ensure drainage). But if it is a case of poor drainage and too much water, I would have expected the armeria to have reacted in the same way probably even sooner. In the plastic pot, poor drainage and too much water could be a possibility... Does it sound likely? Sorry, not much help...