Yeah a green egg layer can lay blueish eggs. With my olive eggers I’m getting eggs more on the dark side, but I’m sure by fall they’ll be lighter and even close to blue, but I just call it light green.
Hens can change shades of colors. But they can’t** switch from one color to another. So brown to light brown is possible, but not white to dark brown.
Edited for the typo. They CAN’T switch from one color to the next.