Chickens can’t change egg color. Blue eggs are particularly genetic. Eggs all start out either white or blue. Each egg will be “spray painted” with a protective coating as it’s being laid. Each bird is limited to one color of coating they produce. It can vary in depth of color as it gets washed out at each seasons end. A green egg, indicative of an Easter Egger, comes from a chicken that lays a blue egg. An olive green egg comes from the same type, a chicken that lays a blue egg. A chicken cannot lay white eggs and later switch to laying blue eggs. It is the color of the protective coating that determines if the egg is blue, green, or olive. Again that coating can only be one color throughout the life of the chicken. Clear coat on blue produces blue eggs. Brown coat produces green eggs. Chocolate coat produces olive eggs. None of the colors can change except for intensity of the coating color. Marans lay white eggs with chocolate coats. Interesting thing about blue eggs is that the shell is blue inside. This means green or olive egg shells will be blue inside also. Have you ever noticed any eggs with blue inside the shell? That would be your culprit although that doesn’t explain the coat. For this reason I believe it will be your youngest bird, that she just started laying.
Hopefully someone with more breed experience can chime in. I can’t seem to call them in cuz this stupid cellphone is acting quirky today.