Just when I think I know my NN flock they go and do something to perplex me. It seems that one of my brown egg-laying girls occasionally lays a green egg. My girls have been especially happy and productive this week with all of them giving me eggs each day...always three brown, one rose and two green. Then suddenly yesterday it was two brown, one rose and three green.
I have no idea who my little chameleon layer is, but have it whittled down to four possibilities. Now if they would just cooperate with my investigation and confess!...LOL!
I don't think you have I e that is switching egg shell colors on you. That is no possible. A hen will lay the same color egg every day that she started with.
Most likely what happened was that one of the green egg layers laid an egg after you gathered the eggs the day before then laid yesterday a little earlier. That has happened to me before.