Welcome to BYC! My wife and I grew up in Texas. Let's start with the easy part of your question first. Your "Americana" chicken is not an "Ameraucana," but an Easter Egger. True Ameraucanas lay only blue eggs, not green, brown, or pink. Easter Eggers are hybrids produced by crossing Ameraucanas or Araucanas (another blue egg laying breed) with other breeds to produce eggs in different colors such as olive, green, beige, and even pale pink and gold. Unfortunately, hatcheries commonly sell them as Araucanas (true Araucanas have no tails) or Ameraucanas (often misspelled as Americana or Americauna by the hatcheries.
As for why your Easter Egger's eggs have changed color, that is really strange. The exact shade of a single Easter Egger's egg may vary, depending on her age and how long she has been laying since her last molt, or because of what she is eating. For example, a hen that lays a light shaded egg before her molt can lay a much darker shaded egg after her molt, but the base color will still be the same (a green egg layer will still lay green eggs). Perhaps this puzzle can be answered with some questions. You have five other chickens. Is it possible that some of them are also Easter Eggers that are laying and that the hen in question is not laying all of these eggs? Are you positive that it is the same bird that has layed blue, green, brown, and pink eggs, because genetically that is suppose to happen? Maybe you could post some pictures of your six birds and these eggs and we can solve this mystery.
Does any other BYC members out there have any other thoughts on this?